I am a new Knitter. I am learning about Increases and decreases. Why would I use either M1L or M1R? Is one to be used on the right part of the pattern and the other to be used on the left part of the pattern?

There are left leaning and right leaning decreases and increases. Which one you use depends on the pattern and which way you want the increase to lean. Check the page with increases under the video tab.