Aperture
Diafragma · Exposure
The opening in your lens that controls how much light passes through.
A wide aperture (low f-number like f/1.8) lets in lots of light and gives shallow depth of field. A narrow aperture (high f-number like f/16) lets in less light but keeps more in focus. The f-number is counterintuitive: lower number = wider opening.
Tools that help with this: