Exposure Lock (AE-L Button) is the abbreviation stands for Automatic Exposure Lock. It is useful using automatic and semi-automatic shooting modes. Where has this information been all my photographing life? So freaking helpful it's not even funny.
Apertura, velocidad de obturación, e ISO, explicados en una sola imagen
Learning even just the basics of photography takes a bit of work and one of the more complex ideas is the relationship between ISO, aperture, and shutter speed. Lifehack has a graphic that helps make sense of it.