The 45th parallel can't be seen, but it's as real as the property line dividing your backyard from your neighbor's. Here's a primer:
The 45th parallel is a measure of latitude. Latitude and longitude are horizontal and vertical grid lines that geographers put on maps to help navigators figure out where they are on the globe. Latitude, the horizontal lines, are actually circles of varying sizes -- the biggest one around the equator and others getting smaller and smaller until they reach the poles.
The 45th parallel is one of those latitude circles.
To know why it's called the 45th, imagine standing in the center of the Earth. Reach your hand straight over your head, and that's the North Pole, 90 degrees latitude. Reach your hand straight out, that's 0 degrees latitude, the equator. Reach your hand half way between those two points, and that's the 45th degree of latitude north. Aha! Alpena.