Suspect is a Bloomburrow keyword that turns a creature into a liability. When a creature becomes suspected, it gets two permanent downsides: it can't block, and it has menace (it must be blocked by two or more creatures). The status sticks until something explicitly removes "suspected" from it.
The biggest misconception is thinking suspect is purely a drawback you inflict on opponents—it's both. Many cards suspect your own creatures as a cost for a benefit, so weigh the menace upside against the inability to block. Also note suspected is a status, not a counter, so it can't be removed by proliferate-style counter interactions or things that remove counters.
In Commander, suspecting an opponent's blocker can punch through a key attack or shut down a defensive wall. On your own attackers, the menace makes evasion easier in multiplayer—just remember those creatures are stuck on offense.