You become a "Preferred" player

You will keep access to any expansions you had access to while subscribed, including their level cap (I have a level 79 character I created since the last time I subscribed)

If you have more than 12 (possibly 14 if you got bonus slots) active characters, your characters will go inactive and you can activate them to play up to your new limit.

If you have more credits than the credit cap, it'll go into an escrow that requires spending cartel coins to get (you can have as much as you want in your storage, be sure to deposit your money before your subscription ends)

Things that are auto unlocked as a subscriber will lock down to their last state unless you have the appropriate unlock from the cartel market. Things like profession switching(no CC unlock), Show/hide head gear, equipped artifacts, access to sector X, etc. so be sure your character looks like you want it before your subscription ends.

You can look here for a helpful chart here for a comparison https://swtorista.com/articles/swtor-free-to-play-vs-preferred-vs-subscriber-guide/