Glossary
Stack
In one line: Putting more players than usual on one site, leaving the others thinner.
What it means
On defense a stack is a bold call: instead of splitting evenly, three or four defenders sit on the same site, usually because you expect the attack to go there. If the read is right, the attackers hit a wall; if it is wrong, the other site falls fast.
The attack can also "stack" a site, meaning everyone goes there together. Either way it is a bet based on information or on the opponent's habits.
An example
The attackers have hit B four rounds in a row. The defense stacks B with four players and leaves one on A. In round five the attack goes B again and runs into four ready defenders.