Typically, it's a credit card "hold" for a certain amount for each night of your stay. Holds are supposed to be released at checkout, but sometimes can remain on your card for a day or two.

High-end hotels will ask for a credit card "for incidentals," and if no credit card is available, they may then ask for a "refundable deposit."

LOW end hotels like this Clar-Qual...property... probably had a single guest cause some damage in the room back in 1999, so their [sarcasm] extremely crackerjack [/sarcasm] management team decided that the one and only solution to that was to demand $100 in cash from everyone who checks in, until the end of all time.

Even if this hotel had at least used the term "refundable deposit," it might have placated most. But $100 for eight hours? At this level of hotel, it's virtually impossible to run up that much in charges in 8 hours. I doubt room service was available; given how paranoid the hotel was, I'm sure the room phone couldn't make long distance calls; they can also turn off the pay movies in the room. What's left? Oh, yeah, that one guest punched a hole in the wall 8 years ago...