Stacking is the act of throwing up gang hand signs. Stacking is usually done to show which gang and sets the individual represents, as well as show who their enemies are. Unless you're part of a gang, it's very dangerous to start throwing random gang signs in areas plagued with gangs, due to the fact that rival gang members might confront you.
Majority of gangs operating in the United States stack, however, there are some exceptions. While Crips and Bloods are known to stack, the Black Disciples, one of the largest gangs operating in Chicago, Illinois, do not stack. Lil Reese, a Black Disciples gang member, says the following in song Beef "I never stack I'm 2-4-14."