A password manager is an application that stores an encrypted list of passwords and is useful if you need to remember a lot of passwords. The encrypted list is unlocked by a master password. Since this master password protects all other passwords, it needs to be particularly strong.
KeePass is a free and open source password manager. Other popular password managers include Lastpass and Password Safe. Some password managers allow for the password file to be synced between devices such as phones and laptops.
If you choose to use a password manager, read and understand the instructions first and start using it gradually, adding a few sites at a time. If your password manager stores your passwords in the cloud they won't be accessible if you don't have internet access.
If you lose your master password, your entire collection of passwords will be inaccessible.