Your exposure mode dial is usually found on top of your camera and is usually filled with letters and icons that allow you to pick how you want to make an exposure.
On a lot of cameras there are a lot of exposure modes to pick from.
Here are a few modes that I’m familiar with and a short explanation of what they do:
M=Manual mode. (This is the mode I always use)
You pick aperture, shutter speed and ISO.
A or AV= Aperture priority or aperture value mode.
You pick the aperture and your camera picks the shutter speed and maybe the ISO.
S or TV= Shutter priority or time value mode.
You pick the shutter speed and the camera picks the aperture and maybe the ISO.
The rest of the modes are fully automatic and are usually represented by an icon. These modes bias toward aperture or shutter speed depending on the mode.
For example:
The exposure mode that is represented by a running stick figure icon (sports mode?) is meant to stop action by picking the fastest shutter speed possible given the lighting conditions. ISO and aperture are picked automatically for you.
The exposure mode that is represented by a head and shoulders icon (portrait mode) is meant to blur the background by picking the widest aperture possible given the lighting conditions. ISO and shutter speed are picked automatically for you.
There is nothing that an auto exposure mode can do that you can’t do in manual mode. I believe that working in manual exposure mode allows you to be much more creative than working in an auto-exposure mode.