Part of macOS
timemachinehelperTime Machine
○ Leave it
Backs up your Mac with Time Machine.
What it is
Helps Time Machine prepare and finish backups.
If you stop it
macOS starts it again straight away. Leave it.
The verdict
○ Leave it. Part of macOS, or something that starts again by itself. You never need to close these.
In Activity Monitor
timemachinehelperTMHelperAgent
From Procession’s guide, in the words the app shows.