The lawn is resting. In this park, below Sacre Coeur in Montmartre, it rests over winter from 15 October to 15 April. This means that you may not step on it, much less sit on it during that period. If you do a policeman or gardener will shout, whistle and gesticulate angrily at you until you step away from the grass. The first time I came across this expression was in Place des Vosges, which is quartered. The gardeners very sensibly allow each quarter to rest in rotation so that the grass gets some time to recover from the pounding of so many tourist feet and is not just a mud or dust bowl.
In autrefois, the lawn in Parisien parks was for looking at only. No lines of desire here - not even a bit of lazy corner cutting permitted. Putting a toe over the edge of the path to take a photo of a pretty garden bed meant being threatened with arrest or deportation (not really, but it must have seemed like that to any non-French speaker exposed to the ferocious verbal style of a passing gendarme).