Have I Been Pwned collects data from known data breaches and can notify if your email address, password, or personal data has been exposed. It also allows you to check if a password has been leaked (either through one of your accounts, or someone else using the same password), using a clever algorithm that doesn’t require you to actually share your password (which is of course not safe), or even the full hash of the password. Rather than try to summarize how it works, I’d invite you to just read the Frequently Asked Questions about the service.
I specifically use HIBP through the excellent Firefox Monitor, which uses HIBP as its data provider. I also HIGHLY recommend using a password manager like 1Password (the one I use), LastPass, or KeePass (there are many others, too). I’ll likely talk more about password managers in the future.
(h/t @jayeless for suggesting this, even though I’ve known about it for years!)