A dead note isn't really a note at all. It's a clicking or popping noise made by plucking a string or strings that have been muted or "deadened" by the fingers of the left hand. The technique is frequently used to play percussive effects on an electric guitar; it allows the player to maintain the rhythm of the solo without actually playing any notes. Dead notes are frequently heard in rock and funk guitar solos.
Play a single dead note placing a finger of your left hand on a particular string over a particular fret. Don't press the string down to the fretboard, just rest your finger on the string. Then pluck the string with the pick in your right hand. You'll hear a clicking sound, which you can play multiple times at various speeds and rhythms.
"Deaden" a note by pressing down the string to the fretboard and plucking the string, but then lifting you finger while keeping your finger in contact with the string. This shortens the note by deadening it before it can ring for more than a split-second. You can then continue to pluck the string with your finger just touching it, adding a series of dead notes.
Play six simultaneous dead notes by resting several fingers of your left hand across the entire fretboard. Strum all six strings with the pick. You'll hear a rough scratching, like scratching a washboard. And, like a washboard, six dead notes can be played in a variety driving rhythms at any point of the solo.
Dead notes are usually woven into solos along with all sorts of other techniques such as hammer-ons, pull-offs, slides, etc.
Knowing how to play dead notes is a useful skill for beginning guitar soloists, because they can be used to dress up a fairly simple melody line with rhythmic effects.
Dead notes are not frequently used in unamplified acoustic guitar solos; a string played this way isn't producing enough vibration to be effectively amplified by the guitar's hollow body.