Orb
An orb is how far from exact an aspect or conjunction can be and still count.
What it is
Two planets rarely meet at exactly the same degree. The orb is the allowed margin: a conjunction within a few degrees is close, one at the edge of the orb is weak. Closer means stronger, and an applying aspect (getting closer) is read as stronger than a separating one.
In AstroFinex
The engine computes exact positions every day, so it knows when an aspect becomes exact and lets it weigh most around that date.