The first is "How Autofocus Works" in which you'll learn the basic differences between "Phase Shift" and Contrast autofocus technologies. Each has benefits and uses in your camera. You'll see discussions of Hybrid methods now too.
The second article is PDF file called "DSLR Autofocus - A Primer". It goes into aspects of autofocus not covered in the previous article such as "Single Shot" vs Coninuous Autofocus.
After you've read these two short article, and do as the second suggests, head to your manual and read it! There will be other issues and nuances specific to your camera that will depend on who made it and what model you are using.
The final link is for those of us with BIG thinking caps! You'd better bush up your Trig for this one. "Applied photographic optics" by Sidney F Ray. It take a bit to load and at first it will look like an error. Let us know how the reading went!