Neither a voltmeter nor an ammeter measures resistance directly; a voltmeter measures voltage and an ammeter measures amperage or current. However, using Ohm's law, resistance can be calculated from known quantities of voltage and current.
Voltage is defined as the pressure pushing electrons through a system, while amperage or current, is the number of electrons moving through the system. Resistance, on the other hand, is a measure of how much a material opposes the flow. To relate the three, Ohm's law states that resistance is equal to the voltage divided by the current. This means, then, that an increase in pressure will increase the amount of opposition to the flow, as will an increased movement of electrons.