A crippled stack is a stack which due to a relatively low size, cannot exercise certain options and is forced to play risky situations.

Crippled stacks are common in tournaments where no rebuy is allowed. Such stacks have less influence in a ring game where a player is free to pad their stack or leave the table.

Relative to minimum bet

At which point a stack is crippled is not strictly defined, but in general it relates to the minimum bet, blinds, and ante. Though considering the average pot size is also a reasonable tactic. Any time a stack slips under the average pot size that player's options will be significantly limited.

The first two options which are lost are the check raise and bluff. A bluff can not be done simply because a bet can not be high enough to sufficiently alter the pot odds and force a player to fold. For the same reason the check raise will likely just become a check call -- some other player will bet and the short stack will likely be faced with an all in call or fold.

Forced play

All games have a slow unavoidable drain on the stack: the blinds and the ante. A player with a small stack size will be forced to stand up even if they never play another hand. Knowing this eventual ante death is coming, they are essentially forced to play hands which they would not otherwise play.