A biconditional is true when both parts are true or both are false: when they have the same truth value.
It is false when one part is true and the other is false.
It is sometimes written as if and only if.
Example: You get cake if and only if you eat your vegetables
This means:
You eat your vegetables ✅ and get cake ✅ → True
You don't eat your vegetables ❌ and don't get cake ❌ → True
You eat your vegetables ✅ but don't get cake ❌ → False
You don't eat your vegetables ❌ but get cake ✅ → False