The name is pretty self-explanatory, really. Paint an army as the challenge runs
One unit each month, photo of unpainted at the start and then painted up at the end. Previous editions were a year long, usually with 2 Real Life cards you can use in case of... well, Real Life interfering. Thus, at the end of a completed challenge you'd have 10-12 units... a respectable army for most factions.
will probably tweak the rules a bit given that he's only planning on 10 months, we'll see.
Note you don't have to paint a unit the way you plan to use it, dividing up 20 Warriors into 2 minimum size units would be fine for example.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.