What's the difference between a secure compare and a simple ==(=)

Github's securing webhooks page says:

Using a plain == operator is not advised. A method like secure_compare performs a “constant time” string comparison, which renders it safe from certain timing attacks against regular equality operators.

I use bcrypt.compare('string', 'computed hash') when comparing passwords.

What makes this a "secure compare" and can I do this using the standard crypto library in Node?