It's pretty fun.
You start by selecting your loadout. You choose a primary weapon (think assault rifles, shotguns, sniper rifles), a secondary (like pistols), and a grenade. Then you choose your Stratagems - these are special drops you can call in - they consist of things like automatic turrets, artillery support, bomber/fighter support, armored mech suits, and special weapons like grenade launchers and heavy machine guns. Typically you're trying to balance what you have with what your teammates have so you don't all end up with nukes and heavy machine guns, though it's usually not the end of the world at least on lower difficulties if you do.
Then you drop into the mission. There are two main fronts spread across several planets. Some are against robots (with ranged weapons and heavy armor and shit), and some are against bugs (think the bugs from Starship Troopers or perhaps Zerg from SC2).
You have a primary objective and a timelimit. Your primary objective could be something like activating and launching an ICBM, evacuating civilians from an outpost, extracting fuel from pipelines, etc. There are also many secondary objectives, such as destroying enemy nests, extracting research data, destroying certain facilities, loading and activating artillery, etc. And then there are many points of interest where you can find resources. Once you've done the primary objective you have to reach an extraction point and call in extraction, wait for the ship to arrive (I think it's two minutes?) and then you have like 20 seconds to load up or you're left behind. In this respect it's like L4D.
As you play you find resources and get experience and another currency that let you upgrade your ship with new facilities - this helps you do stuff like call in certain stratagems more quickly, get more ammo with your stratagems, etc.
You also get war medals that you use to unlock new gear, everything from cosmetics to new weapons. These are unlocked through War Bonds (think Battle Passes), but you can grind out war medals through personal and community objectives as well as find them in missions.
It's generally a really great time.
Oh and you get a certain number of "reinforcements" per mission, so it's not like you'll play for 30 minutes then die then just be screwed out of rewards. Your teammates can call in a spawn beacon and you will respawn.
RS3: UltimaSuende
https://letterboxd.com/BMovieBro/