How is this Phone Number Validation work?

so I'm teaching myself jQuery recently and I just came to the chapter of AJAX. There is an example showing how to check the validation for phone number and I don't know what's going on and how it works. Here is the snippet of code:

// Validate a phone number field
$( "#form" ).submit(function( event ) {
    var inputtedPhoneNumber = $( "#phone" ).val();

    // Match only numbers
    var phoneNumberRegex = /^\d*$/;

    // If the phone number doesn't match the regex
    if ( !phoneNumberRegex.test( inputtedPhoneNumber ) ) {

        // Usually show some kind of error message here

        // Prevent the form from submitting
        event.preventDefault();
    } else {

        // Run $.ajax() here
    }
});

What I don't understand is this:

var phoneNumberRegex = /^\d*$/;

I did some research for phone number validation and it all has something like this: a pair of "/ /" and some stuff between them and I've never seen this before, anyone can explain what is going on here and why it works? Any help will be appreciated, thank you!