what can i do with steam trading cards?

[Sandbox Enjoyer Jerry]

i got 4 among us trading cards collected but idk what to do with it?
i also got duplicate undertale trading card but i am going to sell it later

[nullable]

You sell them, gem them, or collect the set to make a badge. The badge gets you some XP which contributes to your Steam level. Most of the remaining cards you'd have to trade for or buy them off the community market. You only get a subset of the set through playing a game.