Access Levels

Ticket: A user or a group with this level of access can only edit metadata of an issue. This includes changing the status of an issue, adding/removing tags from them, adding/removing assignees and every other option which can be accessed when you click "Edit Metadata" button in an issue page. However, this user can not "create" a new tag or "delete" an existing tag because, that would involve access to settings page of the project which this user won't have. It also won't be able to "delete" the issue because, it falls outside of "Edit Metadata".

Collaborator: A user or a group with this level of access can do everything what a user/group with ticket access can do + it can commit to some branches in the project. These branches are defined here using their name or a pattern and needs to be comma separated.
Some examples:

Commit: A user or a group with this level of access can do everything what a user/group with ticket access can do + it can do everything on the project which doesn't include access to settings page. It can "Edit Metadata" of an issue just like a user with ticket access would do, can merge a pull request, can push to the main repository directly, delete an issue, cancel a pull request etc.

Admin: The user/group with this access has access to everything on the project. All the "users" of the project that have been added till now are having this access. They can change the settings of the project, add/remove users/groups on the project.