What is a resistance?

Resistance is the opposition to flow or movement.
Resistance in a a mechanical system could be friction, wind, gravity.
Resistance in an electrical circuit is the limiting or opposition to current flow. It is measured in Ohms.

Ohms law is:

V=I/R
V=Voltage (volts)
I=Current (amps)
R=Resistance (Ohms) Resistance in an AC circuit is called Impedance (Z)