is dessus also an idiom?

Question

in the phrase "Attends, nous sommes en train de travailler dessus."  I don't understand the use of dessus. Is there another meaning than above or is this another idiom? Thanks

Cécile Janty-Davies · Accepted Answer

Hi Lana, 
We are going to change this question as it has been decided that it is too complicated for an A2 level but an explanation just for you -
In French you use 
travailler sur quelque chose ( un projet ) = to work on something ( a project ) 
So if someone asked:
Tu/vous as/avez travaillé sur ce projet ? = Have you worked on this project? 
You might answer 
oui, je suis en train de travailler dessus = yes, I am in the process of working on it 
Hope that helps!