Hi Susan,
I'm glad to help here! I'm posting the technical definition below, but for the purposes of selecting the resistance of an atomizer, a lower number will give you more power (which translates to vapor production and heat), assuming the voltage remains the same. The inverse is true as well; a higher number will give you less power.
http://www.onlineconversion.com/ohms_law.htm
Electrical resistance is the repulsion of a current within a circuit. It explains the relationship between voltage (amount of electrical pressure) and the current (flow of electricity). With more resistance in a circuit, less electricity will flow through the circuit.
https://simple.wikipedia.org/wiki/Electrical_resistance
Best regards,
Brad