A ghost note is a note sung or played more softly than other notes. It is often "felt" more than heard. A ghost note is indicated by brackets placed either side of the note. The amount of "ghosting" is up to the individual musician. A ghost note can be anything from about half the volume of unghosted notes right down to barely audible.
Listen to the recording to hear the effect of ghost notes and then imitate what you hear. A space has been left on the repeat of the recording for you to sing this example with the band. Once you have the feel of ghost notes, try using them when improvising.
As with other expressions and articulations, it is a good idea to practice ghost notes with scales and arpeggios. Here is the G Blues scale using ghost notes on the beat. Once again, there is space on the recording for you to sing on the repeat.
This musical line also uses ghost notes on the beat. As with the previous examples, listen to the recording and imitate what you hear.