It is the panel that goes alongside your tub. It is usually an acrylic bath panel and basically functions as an access panel so that you can get to the plumbing under the bath still, but not have it on display. You might want to replace the side panel of a bath if yours has got damaged or is old. It might be that if you have an old tub you aren't able to find a matching and fitting side panel so making one is the next best thing and means that you won't need to replace the whole bath. Bathtubs come in all different sizes, standard baths are a myth I think! You can make a side panel to the exact measurements if you follow this tutorial. It is expensive to buy a new tub, especially if the tub itself is in good condition and it is only the side of your tub that needs to be replaced.