To check that you don't have any metal knives or similar weapons hidden on you, you have to walk through a doorway which has special machinery that can tell if there is any metal.

If you have coins in your pocket, it is a good idea to take them out before walking through the door. If you can't get through without making the alarm go off, a guard will take you away and search you.

At some airports, they use a hand held scanner to check for metal.