I believe this is what the cylindrical black plastic thing on the back of the engine bay is called. What exactly does it do and how? Also where should it be connected up and what happens if it fails? How can it be tested?

Alot of questions I know but hope someone can help

 

It's connected to the fuel tank, and allows fumes from the tank to go through it and be burnt inside the engine, all for eco reasons.
tabetha

 

So no issues if the vac line is missing?