A cantilever sliding gate sits on top rollers under its rail. Without an anti-lift bracket, someone could lift the gate up and off the rollers and walk away with it. That is a security problem.
An anti-lift bracket is a steel hook or plate fixed to the gate that wraps under part of the roller housing. It blocks any vertical movement of the gate while still letting it slide freely horizontally. Most cantilever-hardware kits include anti-lift brackets as a standard line item.
For higher-security commercial gates, two anti-lift brackets at different points along the rail are common, plus a locking pin or anchor that engages once the gate is fully closed.