Booleans are either true or false. Other types can convert to bool by using the ! (not) operator or the bool function.

The bool() documentation contains a helpful example table to explain how different types are converted to bool.

This code creates a bool property is_the_earth_flat to collection stuff:

.is_the_earth_flat = !true;
Function Description
bool Convert a value into type bool.
is_bool Test if a given value is of type bool.