A population pyramid graph shows the population of a particular country or city. There are a few different kinds of population pyramid graphs that can happen. One is expanding. This is one that looks like a normal triangle, where the bottom is wider than the top. This is when the population has a big boom - many babies are being born and not very many are dying.
A stable pyramid graph actually looks more like a rectangle than a pyramid. This is when the population is staying relatively stable - only a few percentage point differences between years on the population counts. The third kind of population is an inverted pyramid. This is when the majority of the population is older, and people are having fewer children at an older age.