With the pin removed, there is nothing holding the lever in position, which means there is nothing holding the spring-loaded striker up. The spring throws the striker down against the percussion cap. The impact ignites the cap, creating a small spark.
The spark ignites a slow-burning material in the fuze. In about four seconds, the delay material burns all the way through.
The end of the delay element is connected to the detonator, a capsule filled with more combustible material. The burning material at the end of the delay ignites the material in the detonator, setting off an explosion inside the grenade.
Then boom boom, you are dead if you try to put the pin back in!