First thing you notice when you look at a Equirectangular image is that it is distorted. That is because it is not intended to be viewed like a normal photograph, it is intended to be viewed through a piece of viewing software. Also, where a normal photo contains only the image data you would see looking in one direction, these images contain the entire 360-degree visual data from a single location.
Think of it this way, when you look around you see the world from a central pivot point. 360 degrees in all directions. A photo that captures full world needs to contain all of the world around the viewer in perfect clarity.
To do this we flatten the round world. Imagine taking an orange peel and pushing it flat on a piece of paper. To make it cover all the flat area you need to pull and stretch it a lot at the top and bottom to match the coverage of the middle. That is where Equirectangular Imagery comes in. It allows us to warp a spherical world and fit it on a flat medium. The viewing software then bends the image back to present the desired area. That is why when you see my photos outside of the viewer, they look like this.
The image above is one I took recently on the small lake in Ca. The lake, that looks curved in the photo, actually runs from left to right. Imagine rapping this image around you so that the left and right edge touch behind your back.
The very top of the image is the spot directly above your head and the bottom is the spot between your feet. As you can see in this photo I am standing under the tree.
While the center remains center, the left and right side are, in fact, the back. Which makes Right and left halfway from the center to the edge on their respective side.
That is why a direct line running from right to left looks like this.
Areas at the very top and bottom are the most distorted as they compensate for the stretching of the sphere to fit the flat format of the medium