Check if your password is already compromised
This tool works by taking your plain-text password, passing it through a SHA1 hashing function, then sending only the first 5 characters of the hash to https://api.pwnedpasswords.com
The API then replies with a list of all the hashes that start with those 5 characters, and your browser compares them with the full hash you originally generated from the password and lets you know if there were any matches.
This way you do not need to send your plain-text password or even the hash of it to the api. This keeps your password safe from exposure to the api server on the public internet.