Yes, depth of field is the "in focus" part of the picture. It comes in small, medium, and large corresponding to wide, medium, and narrow apertures. The "acceptable focus" area is normally a 50/50 split in front of and behind the point of focus.
As you make the aperture smaller to get more in focus, you are cutting down the amount of light that comes through the lens so you need to either increase your ISO or lengthen the shutter speed or both.
Increasing the ISO brings in more digital noise (grain) after a certain point, where depends on your camera.
Making the shutter speed longer increases the chance of camera motion blur due to your shaky hands and/or motion blur of the subject moving (not usually a problem with landscapes). VR/IS can help a little with your hands; tripods are the best but not allowed everywhere.
Everything in photography is an compromise.
For maximizing your depth of field for landscapes, take a look at articles about "hyperfocal distance".