@Jo620 It means what it says- that the washing machine is either a part of the regular amenities, or that you charge a separate fee for the use of it. Whichever you decide, you should make it clear in your description and in messaging with guests.
If the machine is in the guest's unit, you really won't have control over whether only long term guests avail themselves of it, nor be able to monitor an added cost for short term guests. It's a bad idea to make rules you can't enforce.
So I would just add a few bucks to your nightly rate to cover guests using it without rules, if you feel that's necessary.
Also, it's a good idea not to let guests wash the sheets and towels- you should be in charge of that. They won't bother to pretreat stains, or make sure they aren't washed with something the color bleeds out of, so you could end up having to throw all the linen and towels out.