I have question about "Positive and Negative Lookahead"

// My Question

I have understood the function of x (? = …) but I still don’t understand its function without the value ‘x’ before it. For example, I have the following code:

// My code

let password = "abc123";
let checkPass = /(?=\w{3,6})(?=\D*\d)/;
checkPass.test(password); // Returns true

What is the function of (? = …)? How does it work in my code?