You should never apply anything - guides - tutorials - tips and tricks - without fully understanding the task at hand.
You should do yourself a favor and research before asking questions espeically for various variable definitions and acronyms used.
Many do that - with excellent results - and only then - when you have something you cannot figure out - then you ask.
It is the root of the file tree and in the context of Manjaro it is the partition used for the root file system.
Yes
You can have as many as is necessary - only one can be active - and that is the one which is selected in the firmware - and you can just use the Windows partition.
The installer simplifies that - what is important is the partition type - which must be of the EFI partition type.
That is because you don’t understand the previous and possibliy because you have been light reading and trying to apply something you don’t fully understand.