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DIN is a German standard for the release settings on your ski bindings. The setting is determined by a combination of skier height, weight, boot length, and skiing ability.
Adjusting the DIN setting either compresses or decompresses a spring in the binding. That compression determines just how much force is required for the bindings to move and release the boot. Higher settings require more force, lower settings require less.
Here is a web page that explains it and shows a DIN chart (about halfway down the page):
http://www.skiernet.com/ski-gear-guide.html
Do NOT adjust DIN settings yourself. Springs flex, wear, and corrode over time, so the numbers on your bindings may not reflect what the spring will actually do.