Every photo balances three settings against each other. Understand the trade-off, and manual settings stop being intimidating.
Aperture (f-stops) controls how much light enters and how blurred your background is — counterintuitively, a lower number (f/1.8) means a wider opening and more blur; a higher number (f/16) means a narrower opening and a sharper, deeper scene. Wide apertures suit portraits and low light; narrow apertures suit landscapes where you want everything sharp.
Shutter speed controls how long your sensor sees light, and whether motion freezes or blurs. Fast (1/1000+) freezes action; slow (1/30 and under) creates intentional motion blur, usually needing a tripod. A useful rule: don't shoot handheld slower than roughly 1/your focal length, to avoid shake.
ISO controls sensitivity to light — raising it lets you shoot in darker conditions, at the cost of visible grain as it climbs.
How they work together: change one, and you often need to adjust another to keep the same overall brightness. There's no single "correct" combination — only the right one for what you're trying to capture in that specific shot: how blurred you want the background, whether motion should freeze or blur, and how much grain you can accept.