It means the pack is "extendible" - usually by means of a cuff at the top. (The numbers typically are given in liters; there are about 60 cubic inches per liter.)
The basic capacity of the pack is usually the first number, and the extension cuff adds the "+" number if you need extra capacity. For example, on the Deuter ACT Lite 50+10 I carried last weekend, the main compartment has a pull-cord running around the top seam; there is then a draw-corded sleeve, about 6 or 8 inches high, above it. For my typical summer load, I fill the pack almost up to the first (50-liter) seam, pull the cord tight, then close the top cord and roll the excess down into the pack. For a winter load, I'll end up filling the pack pretty much to the top of the sleeve, tighten both cords as much as possible, then close the lid; I've now filled 60 liters (50+10) of space.
Usually, in an "X+Y" pack, the lid compartment will be of the "floating" variety: that is, there will be a couple of webbing straps on the back that allow you to raise or lower the lid to fit the normal or extended configuration. (A floating lid may or may not be removable.)
It's a handy feature - I have one Deuter pack that works year-round, but I have two Osprey Kestrels: a 48 liter for summer, and a 58 liter for winter. There are advantages to both setups; separate packs mean you're not trying to shrink the pack to fit smaller loads, but a single pack saves a couple of hundred dollars and eliminates the experience of "I forgot to move my rainsuit from the smaller to larger pack before the trip. Oh well, maybe it won't rain long." (Not that anyone I know ever did that.