Cantilever sliding gates sit on top rollers under their cantilever rail. Without an anti-lift bracket, you could in theory lift the leading end of the gate, swing the trailing end up, and walk the gate clean off the rollers. That's 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.