Low Resistance Coils
Lower resistance means a shorter or thicker coil. Higher resistance means a longer or thinner coil.
Its basically the literal amount of resistance the current encounters getting from point A(the positive connection) to point B(the negative). Thicker wire means less resistance because it allows more current through at any given time, and thinner wire is the opposite because it allows less. Smaller distance for the current to travel means less resistance encountered along the way, which is why length makes a difference too.
On VV batteries, you can pump up the voltage which in turn increases the strength of the current. On low resistance(like 1.5 or less) too high of voltage means too much current for the wire to handle well, and it can burn your juice. So if you like 4.7+ volts you're better off going with higher resistance, like 2.0ohms or more. Lower voltage with 2.0 or more ohms will usually give slightly less, cooler, vapor, where super low resistance(think 0.5ohms) will give off a warmer, thicker vapor. With mechanical mods and non-VV batteries, you usually only have 3.7volts as an option, which is why the lowest resistance possible is preferred.