irm is short for Invoke-RestMethod. It will download a script from that website.
iex is short for Invoke-Expression. It will run the script.
So this is a way to run a bunch of commands from that website. That's a pretty powerful operation - it's enough to take over your machine if the script is malicious. It's safe if and only if the operator of that site is trustworthy. That's something you will have to decide for yourself.