I'm talking about a standard feature in MS Project which allows to collapse tasks at multiple Indent levels. It's called Outlining Tasks.
It works something like this:
When you select Outlining Level 1 - All the Tasks at Level 1 Indent will collapse
When you select Outlining Level 2 - All the Tasks at Level 2 Indent will collapse
and so on..
If this is not clear please watch this video: https://www.youtube.com/watch?v=jzHNHW5Xrzs to learn more.