Its a chain that runs on a bar attached to the head of the saw. The chain should not move at correct idle, after warmed to operating temp and at proper mixture. You need to figure out where the saw is set at and set it up properly as to mixture and idle, as already mentioned by others.
The bar needs to be straight with no burrs in the track where the chain rides, and not gummed up or pinched either.
The clutch mechanism needs to be the correct one for your model saw, and the chain must match the bar and saw it's used on, as well as being properly tensioned, but not too tight, or loose. Too tight a chain will damage the engine bearing when the saw cools, and the chain should be loosened after being run to reduce this risk of damage. This is stated in one of the various Stihl manuals, I've owned over the years.
So, after having made all the adjustments suggested so far, if you still have a running chain at idle, your carb's functional ability to reach correct idle may be compromised by any number of issues.
Report back where you're after doing what's already been suggested, then let's see where you're at.