Do you mean the "się"? "Chłopiec się myje" = "The boy is washing himself", but "Chłopiec myje okno" = "The boy is washing the window" - he is not washing himself, hence no "się" (it's similar to Spanish: "lavarse" vs. "lavar la ventana").
yea I know that, but why is this in the reflexive section if there is no reflexive? only to show contrast between myć and myć się?
Probably yes, to show the contrast. It's pretty frequent in Duolingo courses to see an seemingly unrelated sentence to show up in a skill, just to introduce a word that is necessary now, but wasn't before.