Its for indigestion, or to be more precise it is a proton pump inhibitor (PPI), which means that it reduces the production of stomach acid. I believe it is one of the five most common prescription medicines in the UK.
I have a bit of a problem with acid reflux, but it is not usually too bad. If I take Viagra, it becomes a big problem (muscles at top of stomach are same type as - well you'll have to guess). So if I take Viagra I also take a PPI.
So I doubt your dad has been prescribed it for cancer as such, but the chemo or some other drug may be giving him acid reflux so this is for controlling it.