Orb — how close still counts
Aspects do not have to be exact.
Two planets are rarely exactly ninety degrees apart; it might be eighty-seven or ninety-three. Orb is how far off an aspect can be and still count. This app uses six degrees (four for sextiles).
Tighter orb, stronger aspect
The tighter the orb, the stronger the aspect. A square half a degree off is visible everywhere in your life; one five and a half degrees off just reminds you occasionally.
This is why "which aspect is tightest" is a good opening question — it is usually the thing the person talks about most.