It's a grey area but it basically is supposed to mean (or can for some people) that there was a death or murder of some sort. Maybe by them, maybe just a gang member or family member died. Could probably have various meanings for whoever gets it, and some people may get it just because they think it looks cool.
a tear drop was/is used to signify each time you've been down ( done prison time ) . i think that a lot of younger people just think it looks cool and get it done without knowing the message they're sending . sooo cool, dumb asses!