I am analysing a database

I didn't understand why reserved space is more important then used space ?

Any explication ? can I get 400 GO ?

What is the best maintenance plan ?

Reserved space is what is currently allocated on disk for your datafile. Used space is what is actually being used to store your data within MDF. 

You can technically get 400GB back by shrinking your data file, but i would not recommend it. Whenever you do a datafile shrink one of the commands it it would run is DBCC FileCompact  that would move data pages around in order to free up the space. Whenever it does that it would fragment your indexes within your database, once indexes get fragmented they would get rebuild, as you should be running some form of index maintenance on your database. When indexes get rebuild it would grow your database file back out to facilitate sorting and rebuilding of the index  ( space requirements would differ depending on rebuild options you chose)

End result you would most likely end up where you started with reserved vs. used space.

HTH