What does ? for fucntion mean in typescript code example

Example 1: typescript default parameter

// Default Parameters
sayHello(hello: string = 'hello') { 
    console.log(hello); 
}

sayHello(); // Prints 'hello'

sayHello('world'); // Prints 'world'

Example 2: function with return type in angular

greet() : number {
    return "Hello, " + this.greeting;
}