A pressured lake means that the lake sees a lot of fishermen or people fishing that lake in a certain period of time.
It has nothing to do with any actual pressure but just that the lake or pond is heavily fished so there may be less fish to catch until you wait a certain amount of time for the fish to replenish.
It can be hard to catch some fish in pressured lakes or pressured ponds.
Basically a pressured lake or pond or other body of water is fished hard by many fishermen.
Lakes in public parks are a good example.
You can catch big bass in public parks by thinking outside the box and using tactics most fishermen don't use.
No guarantees though since catching big bass from the shore in public parks is significantly much more challenging than catching a big bass from a boat.
If you want to increase your chances of catching fish it's best to avoid pressured lakes or pressured ponds or at least avoid them until the fish replenish.