What is a manul slave flash for a camera?

Answer 1

A slave flash is a secondary flash gun that is activated by the camera's own flash (or the one that is attached to it by hot-shoe or cable etc). It is used generally as a fill-in or for highlight work etc. As the primary or master flash goes off a sensor in the slave flashes causes it too to fire.
If this secondary flash calibrates itself (ie works on automatic exposure) then it is an automatic slave. If however you have to "dial-in" the amount of flash yourself then it is a manual slave.
Hope this helps.