What happens when two people open the same job, and why read-only is the answer.
A network location means several people can reach the same projects. Two people editing one project at the same time would mean two copies saving over each other, so a project in use is locked to whoever opened it.
It opens read only, and the title says so, naming who has it: (Read Only, open by ...). You can look at everything and measure nothing back into the file. When they close it, reopen it to edit.
The message tells you who, on which machine, and since when, so chasing it is a phone call rather than a mystery.
Where the app cannot tell who holds a lock, it says "another user" on "another computer" rather than inventing a name. A lock with unknown details is still a real lock.
If the project is open in another tab of this app, close that tab first. Two tabs on one project would save over each other exactly like two people would.
An app that closed unexpectedly, or a machine that was shut down or disconnected, can leave a lock behind with nobody actually working. The app offers to open it for editing anyway.
Only take a stale lock when you are certain the other session is genuinely gone. Two copies open and saving over each other lose work - that is the whole reason the lock exists.
Neither leaves the project in a half-opened state; the app reports and stops.