What is a service charge at a restaurant? Is that like a tip?

I took my mom out to a restaurant and the bill came out to like $137. Then they took 18% "Service charge"





Is that a tip that goes towards the waiter basically?


I still put on another $20.





I was confused and now I am worried that that was too little but can;t go back in time.





Im confused as to what that was...A service charge was like a tip they already took out themselves?|||they are charging you for their service.. so its kind of like they force you to pay a tip