"Oh, something really silly . . . She said he was always trying to help her through the portrait hole, like she couldn't climb in herself . . . but they've been a bit rocky for ages."
-- Hermione explaining Ginny and Dean's breakup to Harry (HBP24)
The portrait hole is the entrance to the Gryffindor common room located behind the painting of the Fat Lady on the seventh floor of Hogwarts Castle. The hole is only revealed when someone gives the correct password to the Fat Lady causing her to swing aside. Gryffindor students must then crawl through the portrait hole to enter their common room:
At the very end of the corridor hung a portrait of a very fat woman in a pink silk dress.
“Password?” she said.
“Caput Draconis,” said Percy, and the portrait swung forward to reveal a round hole in the wall. They all scrambled through it — Neville needed a leg up — and found themselves in the Gryffindor common room, a cozy, round room full of squashy armchairs (PS7).
From the inside, Gryffindors push open the Fat Lady’s portrait and crawl out through the hole (PS9 ff). At times, students get in each other’s way trying to scramble in or out. This kind of minor kerfuffle contributed to Ginny and Dean breaking up in Harry’s sixth year (HBP22).