Orb is how far, in degrees, an aspect sits from its exact angle. 0° is perfectly exact; a tighter orb means a stronger, more defining contact. Orb is the single best proxy for how strongly a transit will be felt.
Tighten the orb and the transit sharpens toward the foreground; widen it and the same contact fades into background colour. The same two planets and aspect can be a defining event or a faint note depending only on orb.
the intensity and prominence of the contact.
Saturn square Sun at 0.2° is the story of the week; at 7° it is barely noticeable.