Basically, both are options for turning DH bindings into something that you can tour in.
Trekkers will go IN your binding, and get remover when you get to the top, allowing you to ski down with the security/comfort of your regular bindings.
MFD is a plate that you mount to your skis, which you then mount your DH bindings onto. These will switch between ski and tour mode, but are always on your ski.
I won't argue any pro's/con's for either, but everyone is very opinionated about both systems haha