One of the great powers of pull requests is that anyone with read access to a repository can fork it, commit some changes to their fork and then create a pull request against the original repository with their changes. There are some files stored in source control that are important. For example, a Jenkinsfile may contain configuration details to sandbox pull requests in order to mitigate against malicious pull requests. In order to protect against a malicious pull request itself modifying the Jenkinsfile to remove the protections, you can define the trust policy for pull requests from forks.

Other plugins can extend the available trust policies. The default policies are:

Nobody
Pull requests from forks will all be treated as untrusted. This means that where Jenkins requires a trusted file (e.g. Jenkinsfile) the contents of that file will be retrieved from the target branch on the origin repository and not from the pull request branch on the fork repository.
Forks in the same account
Bitbucket allows for a repository to be forked into a "sibling" repository in the same account but using a different name. This strategy will trust any pull requests from forks that are in the same account as the target repository on the basis that users have to have been granted write permission to account in order create such a fork.
Everyone
All pull requests from forks will be treated as trusted. NOTE: this option can be dangerous if used on a public repository hosted on Bitbucket Cloud.