Wasting time and energy. Being generally unproductive even though you have a lot of very important stuff to do.
Say your light bill is due by 5:00 PM or they're going to turn off your power. A friend comes by at 4:30 PM and sees you napping.. He'd say, "Dude, you better quit congressing and pay your power bill."
Or... you're just hanging out on Sunday morning and a friend calls and asks what you're doing. You'd reply, "Nothing man, I'm just congressing."