A password manager is a software in which you can store all your passwords. It basically stores all your data in some file or database, and keeps that encrypted. All the passwords can be accessed be one single master password. This ensures that you just need to remember one password, and using that you can access all your passwords.
Here is a free KeePass Firefox addon to access passwords without KeePass application. Access passwords from KeePass database stored on Dropbox, G-Drive, One→