Part of macOS
mdsSpotlight
○ Leave it
Runs Spotlight search and decides what to index.
What it is
The heart of Spotlight. It keeps track of files and hands work to the indexers.
Why it might be busy
After a macOS update, a big copy, or connecting a new disk. Usually 10 to 30 minutes, longer after an update.
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
mds
From Procession’s guide, in the words the app shows.