Adjustment range refers to the minimum and maximum lengths an adjustable pole can be set to — it only applies to telescopic and folding poles, not fixed-length ones. The range matters because different terrain and situations call for different pole lengths, especially in the backcountry.
For backcountry touring, look for poles that adjust at least 20cm. You'll want them longer on the uphill for better reach and propulsion, and shorter on the downhill for cleaner pole plants. Common ranges include 100-125cm, 105-135cm, and 110-140cm. The key is making sure your ideal fixed-length pole size falls somewhere within the range — if you normally ski a 115cm pole, a 100-125cm range works great, but a 105-135cm range gives you more flexibility for steep ascents.
Ski mountaineers who stash poles on their pack for steep climbs should look for ranges that go even shorter. Also, keep in mind that folding poles often have a more limited adjustment range than telescopic ones, so check the specs carefully if versatility is your priority.