What is pH? is a measure of acidity or baseness in chemistry. It is roughly equal to the negative log of the concentration of H30+. H30+, otherwise know as a hydronium ion, occurs naturally in water. Every body thinks water is just H20. And in one sense it is. As with most things in chemistry, there's a little more to it though. Water, is a mix of H2O, OH− and H3O+. The last two of these exists in very small amounts, but they are there.
2 H2O OH− + H3O+Water molecules are continually bumping into each other, giving away and receiving protons (H+). H3O+ has an extra H+, making the molecule positive, so it tends to give away it's extra H as soon as it can, it becomes a proton donater, an acid. In plain old water, there is, on average, equal amounts of OH− and H3O+, which means it all balance out and you get a neutral solution.
When something else is in the solution though, it can disturb the balance. If something occupies the OH− molecule, there is nowhere that the H3O+ to get rid of it's extra it's proton. The solution now has a greater concentration of of H3O+ and thus becomes acidic.
All this is important, trust me. Acid has many roles play in the process of making cheese, the next post will show the first of it's roles: curdling the milk.