Barlow lens
Also called: Barlow, Powermate, focal extender.
A lens that goes before the eyepiece and multiplies its magnification, usually 2× or 3×.
A 2× Barlow turns a 10 mm eyepiece into the equivalent of a 5 mm, effectively doubling your kit. Good Barlows add little light loss, and they keep the comfortable eye relief of longer eyepieces. Tele Vue's Powermates are a refined (telecentric) kind.
A Barlow is most useful when it fills a gap — a power none of your eyepieces gives. If it only duplicates one you have, the eyepiece alone is simpler and has less glass.
In AstroPilot
AstroPilot suggests a Barlow only when it gives a power your eyepieces can't on their own.