The Lock action has the ability to secure the record from any changes being freely made. If you have the ability to Lock a Request and/or Document, then your user account belongs to a Permissions Group that was provided this ability. 

Please be aware that other users within your same Permissions Group or users with higher levels of permission than yourself can freely unlock the record and make changes as needed.

Any user in a Permissions Group that does not have the ability to Lock records will not be able to make changes.