An infestation is an invasion of insects on a place like a house or an attack by insects on a plant.
If you're scared of bugs, you're not going to like learning about an infestation, which is a huge amount of insects going where they're not supposed to be. If a house is full of cockroaches or other bugs, that's an infestation. One or two bugs doesn't count: an infestation means lots of bugs are invading, and it's a huge problem. Also, when plants are attacked by lots of swarming insects, that's called an infestation or plague.