The L Button

The L Button is the most used button in the game. In general, you want to always be holding L.

The L Button locks your speed at a constant value of 11.8. You have a higher speed when walking without holding L, but you don't travel as far. The L button more importantly allows you to have more control of Samus. Just by holding L, you can move her in any direction without having to turn. This also makes movement in the air more fluid.