Have you ever looked at a globe? It's a round ball that shows the whole world. The 45th parallel south is a line that circles around the world, but it's not like any line you see on the globe.
Picture the globe as a big pizza, the 45th parallel south is one of the lines that you might cut the pizza into. If you cut the pizza into lots of pieces, each of the pieces will have part of this line on it. This line is like a belt that goes around the Earth, about halfway down from the top.
The reason this line is important is that it marks a spot where the Earth's surface gets colder. You see, the world is tilted as it rotates around the sun. The area around the 45th parallel south is a pretty chilly place because it's so far away from the sun's heat.
So next time you look at a globe, imagine a big belt running through it, and know that it's in a spot where it's chilly.