Probably just relaxing. My girls will 'take a load off' from time to time and just sit on the ground. If they are doing it constantly and all day long, and if their feathers are puffed up and they are lethargic, then I'd worry. But if you see them doing this, but they are up and about when you check on them later, it's probably nothing. What I do is if I suspect illness but it isn't obvious - just a suspicion - I observe them over the course of the day and even into the next few days. I have a Polish girl now who'll just stand around with her wings slightly drooped and looking a little under the weather - while standing in the run and while on the perch outside the coop, but I'm not super concerned because when I get home from work and go out there making my arrival known, she'll sprint across the run and sometimes scream dramatically (so I take that as a sign that she's probably okay but perhaps just suffering the molting blues).