Mode A (Aperture-Priority Auto) - Nikon D3300 Reference Manual

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.
Large apertures (low f-numbers, e.g. Small apertures (high f-numbers, e.g.
f/5.6) blur details in front of and f/22) bring the foreground and
behind the main subject. background into focus.
To choose an aperture:
1
Rotate the mode dial to A.
Mode dial
106 P, S, A, and M Modes