What is Depth of Field?
Depth of Field is how much of the photograph is in focus. Everything that is in focus is in the Depth of Field. The entire blurred foreground and the blurred background is not in the Depth of Field. Using a small f/16 aperture creates a lot of Depth of Field. Using a large aperture like f/2 which allows a lot of light in creates a very shallow Depth of Field. What is the best Depth of Field? That depends on what you want to express in your photograph!
To get two people in focus where one person is closer then the other, try focusing in between them and try to use a small sized aperture. The smaller size aperture will increase the Depth of Field and hopefully both people will be in focus. The farther away people are, the deeper the Depth of Field. If you find the Depth of Field isn't great enough, and you don't want to or can't use a smaller aperture, just back up some to increase the Depth of Field.
Some cameras have a “Depth of Field Preview” that allows you to see what the actual Depth of Field is by pressing a button near the lens. If the aperture is small like f/16, the Depth of Field Preview will darken the viewfinder. The reason for this is the aperture normally stays all the way open until the shutter button is pressed, then it shuts down quickly before the shutter fires. This allows the viewfinder to be brighter with the aperture normally wide open. This also makes your viewfinder brighter with “faster” lenses that have larger apertures. If the Depth of Field Preview is pressed and everything looks the same then probably the aperture setting is wide open and so the aperture is not stopping down any because it doesn't have to.
The Hyper Focal Distance refers to having infinity in focus, and then whatever else that is in focus that is closer. The Hyper Focal Distance changes with the aperture f/stop setting. Smaller apertures like f/16 create more “Hyper Focal Distance”. So with Hyper Focal Distance infinity is in focus, and so are some things that are closer.