The photography term 'Depth of Field' refers to how much of the photograph is in focus. Depth of field is achieved by using the aperture (f-stop, the opening in the lenses that lets the light hit the film plane/sensor to create the photo). The smaller the aperture the more depth of field. The depth of field in the photo below is shallow or narrow, only a little of the flower (Cherokee Sunset dogwood) is truly in sharp focus.
The photo below has deeper depth of field, more of the flower is in focus. When taking close-ups like this the depth of field is going to be narrow even at the smallest aperture opening.
Here's a part of a handout that I have for the Photography classes I give that helps explain f-stop.