Pink & Whites refers to the colors used in a nail enhancement. When you get a set of Pink & Whites, it is normally done in acrylic or UV gel. Acrylic/gel comes in many, many different colors. Pink & White full sets refer to the pink being at the base of the nail, and the white being the tip - or the extension of the nail.
What is a fill & backfill?
A fill is when the tech fills in the area that has grown out and away from the cuticle area leaving a space between the acrylic/gel and the natural nail that is visible. This space is filled during a fill. A backfill is typically done every other fill. This is done on P&W nails. As your natural nails grow out, so does the nail enhancement and with enough growth, the 'white' extension moves outward and begins to look out of place. The tech essentially corrects that by filing off most of the white along with some of the pink, and then reapplies the white while at the same time - moving the 'white' back to it's original position on the nail. This makes the nail look more in balance. When the white grows out past where your natural free edge is supposed to be looks unnatural after a while. So, that has to be repositioned, and moved back to where it should be. A backfill is also often referred to as a rebalance.
Do I need to get a full set every time?
No, you won't have to get new sets applied very often at all, typically two to three times a year.