I'm sorry, but some apparently need to hear this in VERY simple terms:

[BumRushDaShow]

60. It's an old-fashioned term for a "summons" - the term isn't only or strictly applied to courts.

In this case, it's a step above a "written invitation" or "request" that Congress normally sends out to ask someone to testify in a hearing. And note that not all hearings are "under oath" either.

Over the past couple centuries, the courts have slowly filled in the pieces of how these are handled when it comes to Congress. At some point the person refusing to comply can be held for "Contempt of Congress", which has associated case law that goes along with that -

Contempt of Congress

Definition

Congress has the authority to hold a person in contempt if the person's conduct or action obstructs the proceedings of Congress or, more usually, an inquiry by a committee of Congress.

Contempt of Congress is defined in statute, 2 U.S.C.A. § 192, enacted in 1938, which states that any person who is summoned before Congress who "willfully makes default, or who, having appeared, refuses to answer any question pertinent to the question under inquiry" shall be guilty of a misdemeanor and subject to a maximum $1,000 fine and 12 month imprisonment.

Before a Congressional witness may be convicted of contempt, it must be established that the matter under investigation is a subject which Congress has constitutional power to legislate.

Generally, the same Constitutional rights against self-incrimination that apply in a judicial setting apply when one is testifying before Congress.

Caselaw

Quinn v. U.S., 349 U.S. 155, 75 S. Ct. 668, 99 L. Ed. 964, 51 A.L.R.2d 1157 (1955).

Fields v. U.S., 164 F.2d 97 (App. D.C. 1947).

https://www.law.cornell.edu/wex/contempt_of_congress

And in order to declare the above, the conditions need to be met. What has generally happened in the past is that as they move forward towards the contempt citation, the refuser has generally started providing something. It has rarely gone as far as the intransigence that we are starting to see with this administration, so the options have rarely been tested to their full extent.

There was an interesting piece about all this written by Bill Clinton's former Chief Counsel Jack Quinn in WaPo (he's been on the cable shows as a contributor). He sortof covered the delicate balance that needs to be taken into consideration -

If contempt of Congress can’t be enforced, then Congress isn’t a co-equal branch

By Jack Quinn
September 24, 2019 at 2:57 p.m. EDT

/snip

“Historically, the House and Senate relied on their own institutional power to not only enforce congressional subpoenas, but also to respond to other actions that either house viewed as obstruction their legislative processes or prerogatives.” In 1927, in a case involving the Teapot Dome scandal, the high court upheld the Senate’s power to arrest and jail a witness, the brother of then-Attorney General Harry Daugherty, for refusing to appear. Recalling Watergate, Jonathan C. Rose, a former aide to President Richard M. Nixon, argued Monday that “to command the attention of the Trump White House, the actual arrest of an appropriate witness or two might well be required.”

If that seems drastic — and I concede it would be appropriate only in rare situations — Congress can and should at least impose stiff financial penalties on misbehaving witnesses. There are logistical impediments, certainly, to the House directing its sergeant at arms to go out and arrest members of the executive branch, and if they tried, it could set up a new constitutional crisis within an existing one.

As Lawfare’s Benjamin Wittes notes, asserting inherent contempt power in this way “hasn’t been deployed in a long time, and it’s not 100 percent clear that courts would tolerate it.” But there’s a risk that runs in the other direction: If the stonewalling of the current administration, and Congress’s acquiescence so far, wind up demonstrating that there’s nothing the legislative branch can do to enforce its power, then it’s difficult to say that we have an effective system of checks and balances — one of the ideas that animates our system of government.

Executive privilege disagreements are fought out in this area of delicate constitutional balance. As White House counsel during the days when then-Speaker Newt Gingrich (R-Ga.) tortured the Clinton administration with specious investigations, I personally engaged in a number of those battles: The House Oversight and Reform Committee once voted to hold me in criminal contempt when we were simply unable to compromise on a committee request for documents. The issue was eventually resolved, in part with the help of a GOP congressman also named Jack Quinn. He would joke that his constituents were outraged that their representative went to work for President Bill Clinton.

https://www.washingtonpost.com/outlook/2019/09/24/if-contempt-congress-cant-be-enforced-then-congress-isnt-co-equal-branch/