A grace note, it's essentially a very quiet note that's only played very briefly
Gear List:
B.C. Rich NT Jr. V (With Seymour Duncan AHB-1 Blackout in bridge)
Electro-Harmonix Metal Muff
Marshall MG15DFX
Jazz III picks
DR strings
Planet Waves Cables
It refers to a ghost note, which is played quite softly. It is called a ghost note, not a grace note, as LepperAffinity, said.
There aren't a lot of conventions in Tabs, but it often means an accidentally played note, something that you hit by accident on the way to another. It could be a grace note or it could have a value. It could be a natural harmonic, though natural harmonic are usually written <5>. Also, you can have a natural harmonic on an open string.