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
You can adjust DIN settings yourself, as most shops will just use the same DIN chart and information anyway when setting it, and will charge you for it. BUT Springs flex, wear, and corrode over time, so the numbers on your bindings may not reflect what the spring will actually do. To have this checked (for instance on older bindings) you need to ask for a BFU test, where a machine physically tests the spring. The spring will also wear out faster if it is set very high (near the max DIN setting on your particular binding, usually a highlighted number), so be aware of this factor.
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