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?|||YES IT SORRTA IS=|||Yes, tipping is already included in your 18% service charge. So it's not necessary to tip again unless the service is exceptional. Normally the 18% is shared amongst all the restaurant waiters.|||Service charge has to be optional and is considered a form of official tipping,,they are very annoying and should be banned,,