The M18 Claymore is a special kind of weapon used by soldiers in the military. Imagine a big kite that is shaped like a rectangle and has a picture of a scary face on one side. This kite is actually a weapon that has a lot of small balls inside it called "shot pellets."
When a soldier wants to use the M18 Claymore, they set it up facing the direction they want to attack. Then they press a button on a remote control that they hold in their hand. When they press the button, the Claymore makes a really loud noise and shoots out all of the little shot pellets in the direction it's facing.
This makes a lot of noise and can hurt anything in the way of the shot pellets. So, it's an effective weapon against enemies that are close by. Soldiers use it to protect themselves and their team from any bad guys that might be around them.
That's basically what the M18 Claymore is and how it's used.