Maturity represents the voting rewards accumulated in a neuron. Each day the network rewards participants by allocating to every voting neuron a portion of the total reward, based both on its voting power at the time proposals were made, and the number of proposals it voted on.
Please note that different tax authorities may take different views on the taxation status of the voting rewards. Neuron owners who receive voting rewards and convert them to ICP should consult appropriate professionals.
For those who wish to compound the voting power in their neuron, the most natural activity is to "stake maturity" on a regular basis. If you wish to liquidate rewards you earn from the neuron and convert them to ICP, you can "spawn" maturity into a reward neuron.