Regular Expressions: Positive and Negative Lookahead, caret before the lookaheads

Tell us what’s happening:
After writing my own version, I think I figured out most the reasons why we need the below code to truly pass this exercise. The one thing that I can’t find good information on is why the caret before the two lookaheads precludes a password with a digit as the first character.
I tried to look back through all the exercises but can’t seem to find anything. Is there some documentation I should read? (Sorry if this is really obvious and thanks in advance.)

Your code so far

let sampleWord = "astronaut";
let pwRegex = /^(?=\w{6,})(?=\D*\d{2})/;
let result = pwRegex.test(sampleWord);

Challenge: Positive and Negative Lookahead

Link to the challenge:

The caret symbol means “the start of the line”. It is used here to make sure that no characters (such as numbers) come before the pattern.