Depth of field is that recognizable quality of a photo that many associate with being more "professional". That quality is the painterly blurry background with a razor sharp foreground of any subject. This quality is controlled by your lenses APERTURE/FSTOP.
For portrait purposes, typically, you want that sharpness to include, if not anything else, the eyes of the person you're shooting. This applies especially to babies as they move so much. The true skill lays in capturing those sharp eyes of little ones. This technique is considered as "Shallow Depth of Field". This is controlled by using a WIDE APERTURE/FSTOP i.e. 2.8 or 3.5.
In contrast, if you're shooting, for instance, a landscape, or group portrait, the goal is to create what's called "Great Depth of Field". Meaning everything in the frame (picture) is sharp in focus. This is controlled by using a small APERTURE/FSTOP i.e. 8, 11, 16, etc.
For this example I love to use the photo legend, Ansel Adams as an example. If you're familiar with his work, you should recall his shots pretty vividly because that's what they were, VIVIDLY DETAILED. Every tree branch, every hillside and mountain peak were RAZOR SHARP.
The first, I used an aperture (fstop) of f2.8 (wide) to create a lot of blur of the wallpaper. The next, I wanted to capture just a little more of the wallpaper, a subtle but visible difference, so I used an aperture of f8.
This technique is really the core of portraiture. Remember that the lower the number (i.e. f2.8) means that the opening of your lens is WIDER. The HIGHER the number (i.e. f8, and above) your lens opening is smaller.
Know that all lenses and camera models have a slightly different range of apertures, but I recommend changing them in whole stop increments as I've shown on the diagram.
Don't feel overwhelmed if the numbers don't process right away, they will.
Try it for your self!