That could be somthing like ganggo, socialhome or diaspora. Make sure it matches the PROJECT variable in gitlab-ci.yml!
This must be the FQDN of the server your repository is on e.g.
Your project slug is the Gitlab user- and repository name separated by a slash e.g. ganggo/federation
We need access to your repository to update merge requests. You can generate one by accessing https://<your-gitlab>/profile/personal_access_tokens. Its necessary that you click on api in the Scopes-Section!
That would mean that tests are triggered if you specify [ci] in the PR title, body, commit-message or if the PR was tagged with ci. If you leave the box unchecked tests will be triggered on every pull-request. However you can skip them by using [ci skip] in the mentioned fields or tag it with ci skip.
If you want to use custom build triggers for opt-in you can specify it here.
If you want to use custom build triggers for opt-out you can specify it here.