Mining is the method how transactions on the Bitcoin payment network get processed and stored. The only reason you would mine is if you wish to invest the significant amount of time, equipment and electricity necessary to do so. Compensation for the mining activity comes from "rewards" in the form of new bitcoin currency that is issued.
The more cryptographic hashing calculations that your hardware can perform, the more likely you'll earn the currency "generation" reward. The reward is issued roughly once every ten minutes. All the miners are competing to become the recipient of that next reward.
Because there is no central authority, the currency is protected by the network. The more mining activity, the less likely an adversary could mess with the payment network's transactions. Now that Bitcoin is among the largest distributed computing networks that exist, corralling the necessary computing power to act as an adversary is becoming something that is more and more difficult.
The more cryptographic hashing calculations that your hardware can perform, the more likely you'll earn the currency "generation" reward. The reward is issued roughly once every ten minutes. All the miners are competing to become the recipient of that next reward.
Because there is no central authority, the currency is protected by the network. The more mining activity, the less likely an adversary could mess with the payment network's transactions. Now that Bitcoin is among the largest distributed computing networks that exist, corralling the necessary computing power to act as an adversary is becoming something that is more and more difficult.