A debt holiday is basically a pause on recurring payments. Housing and utilities have been the main focus of the push, because they're essential and because everyone knows that merely suspending payments so that people get hit with 3 or 4 months' worth of expenses all at once when the moratorium is lifted is only going to postpone the crisis. Most households would never be able to catch up. Student loans have also been part of the conversation, simply because they're under the direct authority of the federal government already and because the consequences of default can be so disruptive.
So the holiday idea is a pause button so that not only are no payments due, there is also no past-due balance stacking up. With mortgages and student loans, it would prolong the payoff period by however long the "holiday" lasts - basically just tacking the missed payments on to the end, a measure that was proposed to ease the foreclosure crisis in '08 but rejected for political reasons. It is less clear how it might work for utilities, and I've read a few plans. I think the easiest is a targeted tax credit for utility companies that would make up for the lost revenue, but I know at least one of our essential utilities are provided by a company that gets held up as an example of corporate welfare because of a near-zero or net-zero tax liability in most years so they may not be convinced to participate by a credit they don't need.