what to do with RSU?

hi everyone, i’m joining a well known tech company later this month as a new grad SWE. in my compensation package, i have an amount of RSU (shares?) that vest in X number of years — this is what the recruiter told me and what my offer letter says, but i’m not really sure what this means? the company’s stock hasn’t been performing that well for the last year but it’s going up slowly, and i was also told that that’s a good thing for new ppl joining the company tho i’m not sure why. any explanation/guidance would be appreciated, thanks!

tldr: i don’t know what RSUs are and what im supposed to do w them