Reactive Power is when the Current flow, caused by AC Voltage applied across a device, results in the Current flow being either ahead or behind the applied AC Voltage.
Reactive devices will store some Energy as Voltage is applied, and they will return that Energy later in the sine-wave... Think of a Spring... You put power into a Spring, then when you reduce or remove the force, such as Voltage, the Spring will spring back returning the energy put into it... No Energy is absorbed, reactive loads return the energy put into then later.
If you charge a Capacitor with a DC voltage, then after the DC voltage connection removed, you put a lamp across that Capacitor, it will make the lamp light briefly, as it returns the stored energy.
In the same way, if you connect a Voltage to an Inductor, such as a Motor, then reduce or remove the voltage, the Inductor will kick back with energy as its magnetic field collapses.
So "Reactive Power" is a means of explaining how the Current seen in a Reactive Load behaves, relative to the applied AC Voltage... Further understanding gets more complicated, and can be explained better, against more specific and direct questions.
"REACTIVE POWER"... Let's clear up one common misconception, the Generators and Grid supplier, do not supply you with Reactive Power... Power is Voltage and Amps.
The power company supplies you with AC Voltage.
What you do with that voltage is up to you and your equipment.
If you put a small (high ohm value) resistor across that voltage, you will draw a small current.
If you put a large (low ohm value) resistor across that voltage, you will draw a larger current.
Similarly your equipment controls the Current, not the supplier... If your equipment is pure resistive, then the Current will be in phase with the applied Voltage... But if you connect an Inductive load, such as a motor, the Current cycle will lag behind the applied voltage... This means the Current AC wave form will be rising later than the voltage rise, bus so it will also be falling later than the voltage rise. This means a certain amount of power will be absorbed by the Reactive part of your load, but then that power will be returned later in the cycle... So, on average, no Reactive power is consumed... It is absorbed like compressing a spring during part of the cycle, and returned to the system later in another part of the cycle, like a spring pushing back.
In Motors, the Reactive part of the power creates a magnetic field, which then collapses and reforms in the opposite direction as the applied AC voltage goes through its cycle... It is this magnetic field that provides the mechanical force between two different parts, resulting in the Motor rotating... Only real Power is consumed, as in the Motor doing mechanical work... Some real Power is lost as heat, in the various inefficiencies as losses.