A personal access token is a string, similar to a password, that you use to authenticate yourself when accessing a system or service. However, unlike a password, you can create multiple personal access tokens and remove them from your account. Personal access tokens provide better security than passwords because they are generated by a computer and can be revoked if compromised.
For GitHub, Xcode will automatically creates a personal access token for you. For the other source control systems, follow these links to create a personal access token.
For Bitbucket Cloud, go to Bitbucket Support: App passwords (app passwords can serve as personal access tokens).
For Bitbucket Server, go to Bitbucket Support: Personal access tokens.
For GitLab, go to GitLab Documentation: Personal access tokens.