This video is a good summary. The embedding doesn't work, so you'll have to click the link instead. Just left the embed on there for the picture. :)
Total Pressure of air can be broken into the partial pressures of all the gases contained in that air. For instance, if the atmospheric pressure is at 760 mm Hg, and 20% of that is Oxygen gas, 80% Nitrogen gas, then the partial pressure of O2 would be 20% of 760 = 152 mm Hg. Partial Pressure of N2would be 80% of 760 = 608 mm Hg.
Here's a long video on partial pressure and gases getting into solution, I didn't watch the entire thing yet but it looks like a good detailed explanation for those who feel they could use more information.