Drainage.

Spider house.

Likely a drain for condensation etc. My window kept pooling up with water, and it turns out the previous resident siliconed over the drains like these

To elaborate on what others have said, with a window that slides open, rain and condensation can run down the sliding part when it’s closed and pool up in the track, eventually leaking inside your house. There’s usually dinner kind of drainage at the bottom that allows the water to drain outside. Often it’s a hole like that at the end of the track, then a little slot on the outside.

It’s for the spiders to lure in the ants. /s