I think the same Qs keep getting asked but turned around a slight bit....

A = Aperture. Larger aperture lets more light in, which means u can shoot at a faster shutter speed (to let the same amount of light in). Ie: instead of shooting at f/3.5, u can open the aperture up to f/2.8 which allows u to shoot faster. shooting faster means u can shoot moving objects with less blur. another feature of aperture is that larger apertures have a shallow depth of field. this means that the background will blur, which is a good thing for most photos, so u can focus on what's being focussed. vice versa.

S = Shutter. Basically the same principle. A and S work together. if u increase A u need to decrease S for same exposure. For handheld slow moving subjects u need to shoot faster than 1/15 to not get blur. for sports and medium moving subjects, it's around 1/300 or faster seconds. To freeze fast moving objects that are moving very rapidly u will need 1/500 or quicker (tennis racquet swinging thru the air). a rally car sits around 1/400 for a perfect freeze, and 1/100 with panning so u can have the wheels still in motion. so this mode is all about freezing objects to stop blur. sometimes ur camera wont let u shoot athe shutter speed u WANT, as there are limitations on available light or the lens aperture can't support it (ie u want to shoot 1/4000 which would mean a very big corresponding aperture - which wont work for shitty lenses).

if u want quality shots in low light, open the aperture big as possible, and if the corresponding shutter is still slower than 1/15 seconds, then u need to get a tripod or raise the ISO.

The following is shot at shutter priority at 1/320 of a second to freeze the person but show motion in the tennis racquet (desirable). the corresponding aperture which the I did not control, was f/5.6. that means there was room to shoot faster speeds IF I needed it, since my lens can go f/2.8. ISO was 280 as it was cloudy and light was at a premium when the sun was behind it.



shooting faster here at 1/640 " allows total freezing of both the person and tennis racquet. the camera set the equivalent aperture to f/4.5, which again shows theres room for a bit faster shutter speed at that ISO.



last example faster still @ 1/800" and aperture maxed out at f/2.8. The camera maxes shutter speed at 1/8000 but since aperture is already maxed, i cant shoot any faster (unless i raise the ISO).