If you remember one thing, remember to associate the number 100 with this unit of measurement. It comes up again and again. Firstly, understand that the metric (systeme internationale) prefic hecto- refers to the number 100. Hecto- is sourced from the Greek language's word for 100. And so, if you measure a length of 100 metres, you have one hectometre (hm).
Ah, but hectares are for area, not length. If you have a square measuring one hectometre by one hectometre, you have one square hectometre. Well the word hectare means the same thing. A hectare is one square hectometre.
OK, so then you get to thinking, if a hectare is a thing, how about and are, so that a hectare is 100 are? Yes that is right. I've never seen anyone bother with ares, or kiloares, or milliares or whatever though. The hectare is not even necessary really, as it could simply be called square hectometre, or square metres and square kilometres could be used instead.
Certainly, when dealing in large enough areas, square kilometres is the standard. And here is the number 100 again: there are 100 hectares in one square kilometre.
How about that are thing again. Well being 1/100 of a square hectometre would give it an area of 100 square metres, since a square hectometre is 100 metres by 100 metres, i.e. 10000 square metres, and 1/100 of 10000 is 100.