A mute is when we place our fretting hand fingers over the strings and then strum. This gives a dry percussive "beat" sound. Don't press down the strings, just let your fingers gently rest on them. Mutes work well in uptempo, funk and rock songs.
A percussive slap is when you take your strumming hand and hit the strings so they slap against the neck. Ed Sheeran has made this technique popular. It's used in fingerstyle and chord strumming to give a percussive tapping sound. It's the guitar equivalent of a snare drum.
In TAB a mute or slap is written as an X.