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

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

Nobody
merge 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 project and not from the merge request branch on the fork project.
Contributors
merge requests from collaborators to the origin project will be treated as trusted, all other merge requests from fork repositories will be treated as untrusted. Note that if credentials used by Jenkins for scanning the project does not have permission to query the list of contributors to the origin project then only the origin account will be treated as trusted - i.e. this will fall back to Nobody.
Everyone
All merge requests from forks will be treated as trusted. NOTE: this option can be dangerous if used on a public project hosted on a Gitea instance that allows signup.