how to add a reference to assembly 'Mono.Android"

I keep getting an assembly error is asking me to add a reference to an assembly but i dont know were its located

error:

The type 'Android.Content.Context' is defined in an assembly that is not referenced. You must add a reference to assembly 'Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=c4c4237547e4b6cd'.

Can someone please Explain.