I was practicing and trying to make an apple pie depending on this source: http://allrecipes.com/howto/perfect-pie-crusts/detail.aspx
In the Liquid section, it's said: "A little bit of acid--vinegar or lemon juice--helps tenderize the dough and prevents it from oxidizing."
What's "Oxidizing"? First time I read/hear this term in cooking?
Please help me learn, I'm still a beginner! Thanks in advance