I have an en-suite power shower and the grout on the bottom 2-3 rows of tiles has breaks in it.

As a result water has got behind and discoloured the ties. As I have spare tiles I intend to remove the damaged tiles and fix new ones.

Is there someting I can apply/paint onto the plaster wall behind to waterproof the surface to protect it ?

Also is there a specfic grout I should use ?

If it's a power shower then i think you need to remove more than 2 rows to get the waterpoofing( tanking) high enough to stop any ingression.......there are various tanking kits on the market...from Mapei gum wps to tanking membranes...

Ideally it requires tanking all the way up! But for it to be effective you will normally need to go up at least a metre

Kev