I agree with others, its usually a sign of poor drainage, which can be improved by aerating the lawn, manually with a solid, or hollow tine tool or the mechanical equivalent or even a every-day garden fork, to make the holes to fill with lawn sand, using the back of the rake, back and fourth to fill the holes.
The existing moss can be raked out with a wire rake or scarifying machine.
Alternatively chemical lawn moss killer, which will only act as a temporary fix and the moss is likely to soon come back.
It seems to me that improving the drainage is a no brainer