Stacking is done in games in one of two ways. Stacking can either be in team based games or in RPGs. The meaning is the same, but the concept is slightly different.
Team Based Stacking
Stacking in team-based games is team stacking. Teams can be stacked in two ways.
If the game does not have a team balance feature, you will sometimes see games with very bad team ratios with tons of people on one team and no one on the other.
Usually regulars on a server know each other and will all join up on the same team. This leaves all the good people on one side and all the bad people on the other. Furthermore, when people have the option to pick a team, they primarily go for the team that is winning, therefore making number stacking and skill stacking go hand in hand.
RPG Stat Stacking
Stacking in RPGs refers to stat stackng (See min-maxing). Stat stacking is when players will load up on stats that are the most beneficial for their class or their role in the game, giving them huge amounts of one or two stats and very low amounts in the other. These characters are designed with one role in mind and usually can really only do that one thing effectively.