A visual as well as verbal gag, which depends on the comic convention of the mouse hole (inverted as a bat hole; compare the balloon dog door in the Sam Gross cartoon here), from which we can reconstruct the presupposition associated with the also in Cat #1′s statement: we have mice.
From the TV Tropes site (with links to two other conventions):
The cartoon mouse’s refuge: a neat arch-shaped hole cut into a wall at the floor, sometimes with a door.
If you have one of these in your house, expect your perfectly triangular cheese wedges to go missing soon.
Often the gate to Mouse World.
TV Tropes notes the mouse hole in newspaper comics:
A trademark schtick of Garfield strips. Sometimes this even extends to windows with pots of flowers under them, mailboxes, welcome mats, etc.
One of each — a lazy Garfield (in a jigsaw puzzle), and Ziggy with his frazzled cat:
TV Tropes goes on to look at the mousehole in animated cartoons, beginning with Tom And Jerry:
The site also tells us that:
Early Mickey Mouse cartoons didn’t show his house, but early comic strips and children’s books did. It had a round, obviously mousehole-like front entrance despite Mickey’s large, non-mouselike size.
The shadow of Mickey Mouse falls on other mouse cartoons, for instance this wordless New Yorker cartoon by Mick Stevens:
and this Carolita Johnson cartoon (which, she tells us, was rejected three times by the New Yorker, with different captions):
(TV Tropes continues with a list of other mousehole appearances in animated cartoons.)
The mousehole is a frequent feature of gag cartoons, like the two just above and this wordless one by Bernard Schoenbaum in the New Yorker:
A corresponding bat hole (or bat-hole or bathole), in contrast, doesn’t figure much in cartoons, though in the real world holes giving bats access to caves, caverns, mines, etc. are commonplace.