A proxy is an indicator for something. For example, the density of a sediment core could be a proxy for porosity. Isotopes are atoms of the same element with differing number of neutrons. There are three isotopes of oxygen. 16O and 18O are present in the air and the oceans.
The ratio of 18O to 16O gives us an estimate of what the ocean water was like through time. Organisms that build their tests from ions in ocean water record the amount of 18O and 16O that was present during their life cycle. The ratio of 18O to 16O in the tests of ocean organisms acts as a proxy for ice volume. How? 16O preferentially evaporates because it is the lighter of the two isotopes. Since it’s easier to evaporate the smaller isotopes, rain/snow are high in 16O. This would make glaciers 16O – rich too. Given the aforementioned facts, ocean water is higher in 18O during glacial periods, and one would also expect ice volume to be greater when 18O values are higher in ocean water and ocean organisms that build their tests from the water. The ratio in the critters also acts as a proxy for ice volume.
On an unrelated note, sunset was stunning this evening. I could tell the ship had gained some latitude because sunrise was after 21:00 tonight! I walked out on the deck at the perfect time to see the sunset, a gorgeous glitter path AND crepuscular rays! Some of my shipmates beat me to it. I think that everyone got a chance to look.