This is the 255th most frequent Romanian word.
sus
"Sus" means "up" or "above" in English.
In this sentence, 'sus' refers to the upper location or top to which the speaker is climbing.
Urc pe scări până sus.
I climb the stairs up.
Here, 'sus' indicates the position of the book being on a higher level, specifically the shelf.
Cartea e sus pe raft.
The book is up on the shelf.
In this context, 'susține' is derived from 'sus' and means to support, metaphorically lifting someone.