Boolean data type can only hold two valid values(true or false). You can only set the true or false values. These values are used a lot in control structures where one piece of code is executed if a condition is true and another piece of code is executed if a condition is false.
Here is a very basic example of declaring Boolean variables:
let booVal1: boolean = true; let booVal2: boolean = false; let booVal3: boolean = 103; // Type '103' is not assignable to type 'boolean'. let booVal4: boolean = "blue"; // Type '"blue"' is not assignable to type 'boolean'.