Imagine two squares sitting side by side, both level with the ground like so:
A simple way to detect if one is hitting the other is to compare the location of each side. They are touching if all of the following are false:
The right square's left side is to the right of the left square's right side.
The right square's right side is to the left of the left square's left side.
The right square's bottom side is above the left square's top side.
The right square's top side is below the left square's bottom side.
If any of those are true, the squares are not touching. But consider a case like this, where one square is at a 45 degree angle:
Is there an equally simple way to determine if those squares are touching?