A smaller part of a bigger thing, like a piece of a puzzle or a part of a list.
Subitems are like small pieces of a bigger thing, and they help make the bigger thing complete.
Examples:
The subitem fits into the bigger thing.
I need to find all the subitems to finish the puzzle.
The subitem is very small and hard to see.
The subitem is like a small secret in the big thing.
There are many subitems in the big list.
The subitem is the most important part of the whole thing.