what does impressions on linkedin actually do?

You can see it per post, there's an impressions count under each one, tap it for the breakdown. Profile-wide it's in your analytics. Weekly numbers jump around a lot so look at a month before deciding anything.

It's how many times your post was shown on someone's screen. Not how many people read it, and not unique people either, so if the same person scrolls past it twice that's two impressions. On its own it doesn't do much. It's more useful next to your reactions and comments. A post with loads of impressions and almost no engagement got shown plenty but didn't make anyone stop, which tells you more than the raw number does.