Your main password signs you in to the Vault and can authenticate compatible Files and Calendar clients. It does not authenticate external email or contacts clients.
Use a long, unique password that is not used for any other site. A password manager can create and store it. Do not send it by email or enter it on a site other than an expected LightDrive sign-in page.
Change the password under Settings → Security → Password & sign-in. A Files or Calendar client that stores the main password must be updated afterward. Vault app passwords and LightDrive client passwords remain separate and do not change when you change the main password.
For third-party apps, a separately revocable client password is usually preferable when one is available because it can be limited to the services that app needs.
If you forget the password, use the recovery link on the Vault sign-in page. Instructions go to the verified recovery email address.