Some users who use password managers such as Lastpass, Chrome, which comes with a password manager, Keychain (iOS/MacOS), or 1Password, may give a warning such as "...a data breach on a site or app exposed your password. Chrome recommends checking your saved passwords and changing your password...". See the image below.
But whatever password management app it is that is telling you is that the combination of the email address and password you're using to log in on bookem.com is the same that you used on a site somewhere else on the web that was hacked at some stage. As a result, your email address and password (possibly name too) is lying somewhere on the web for anyone to copy and try on sites on the web to see if they can get it. This is exactly how your password manager knows this - they download and store all hacked email address/passwords out there so that their customers can be given this warning!
In other words, your password manager is recommending that you change your password that you're using on bookem.com so that someone who has access to whatever site was breached can not use those credentials to access your data on bookem.com.
To prevent this from happening to you, always use a different password for each site you register with, and use your password manager to lock it away and access it and all other passwords with your one password (hopefully much more secure and not involved in a breach!) used to lock up your password manager safe.
For more information, visit the following: Why you need a password manager