Packed length is how short a pole gets when collapsed or folded down. This only matters if you need to stash your poles in a pack or suitcase — for regular resort skiing with fixed-length poles, you can ignore it entirely.
If you're touring, though, packed length becomes important. Folding poles break down into short sections, typically 35–45cm, which easily fit inside a backpack for steep bootpacks or ski mountaineering. Telescopic poles only collapse to about half their extended length (50–65cm), so they'll need to strap to the outside of your pack. Fixed-length poles can't be shortened at all.
The main thing to watch: don't assume telescopic poles pack as small as folding ones. If you need compact storage — for air travel, helicopter trips, or fitting poles inside your touring pack — go with folding poles. Otherwise, telescopic poles are fine for sidecountry or mellow tours where you just strap them to your pack exterior.