It means the manga is good and heart warming all in all. No rape, harassment and complicated turn of events. Just pure mangas that will make you feel good after reading :)
This is an example of a wholesome manga so that youll get what I mean
http://www.mangago.me/read-manga/futari_hyakkei_kaze_no_owari_haru_no_hajimari/
It can also include Slice Of Life, everyday scenarios that are positive. In short, "wholesome" mangas are very clean, feel-good works that make you feel positive. These are often simple and relatable scenarios (though not always), and centers on explorong themes like self/identity, family relationships, growth, etc.
hi...im very confused with people saying "wholesome manga"..what is it means??