Two planets are in aspect when they are a certain number of degrees apart, give or take a certain number of degrees. Those "give-or-take degrees" is the aspect orb.
When two planets are within orb of an aspect, and are moving closer toward the aspect being exact, they are applying. When they are moving away from being exact, they are separating.
Every astrologer has a different idea of what to use for orbs. Not just how many degrees to allow before the orb is too wide, but also many astrologers prefer to use a wider orb for applying aspects and a shorter orb for separating aspects. You have to play around with it to see what works best for you.
Examples of orbs for major aspects that are commonly used:
- Conjunction 0° give or take 8° to 10°
- Sextile 60° give or take 4° to 6°
- Square 90° give or take 6° to 8°
- Trine 120° give or take 4° to 8°
- Opposition 180° give or take 8° to 10°