Nikon D3300 - Manual (Page 126)

106 P, S, A, and M Modes
Mode A (Aperture-Priority Auto)
In this mode, you can adjust aperture to control depth of field
(the distance in front of and behind the main subject that
appears to be in focus). The camera automatically adjusts
shutter speed for optimal exposure.
To choose an aperture:
1
Rotate the mode dial to A.
Large apertures (low f-numbers, e.g.
f/5.6) blur details in front of and
behind the main subject.
Small apertures (high f-numbers, e.g.
f/22) bring the foreground and
background into focus.
Mode dial