Visibility Expression is used to control the visibility of field/panel. The return value of visibility expression should be a boolean(true/false), where true says that the component should be visible and false says that it should be hidden.
Suppose, you have two fields A and B and say, you want to control the visibility of A based on the value present in B. In such case, the visibility expression would look like this,
// suppose we want to make the field A visible only when the value of // field B is set to "female", then the visibility expression of field A // would look something like this B.value === "female"
Also, with the expression above, the initial visibility of field A would be hidden, assuming that B.value is not set to "female" initially.
Visibility Expression can be found in field/panel dialog configuration.
The "help page" for all the "expression" supported would be available soon.