A ghost note is the opposite of an accent. It is a note played softer than other notes. A ghost note is indicated by brackets placed either side of the note as shown below. The amount of "ghosting" is up to the individual player. A ghost note can be anything from about half the volume of unghosted notes right down to barely audible.
As with other expressions and articulations, it is a good idea to practice ghost notes with your scales and arpeggios. This example demonstrates the G Blues scale using ghost notes on the beat. Space has been left for you to play by yourself on the repeat.
Here is a riff which makes use of ghost notes off the beat instead of on the beat.