This tag should be used for questions about the function, the reason for, and the underlying concepts of mining.
Mining theory encompasses two main aspects: why mine and how to mine.
The "why" is usually answered with two reasons: to make money and to help the blockchain move forward. Miners are generally considered to be greedy and that all decisions that miners make should be based upon greed. Making money is the primary reason for most miners to mine Bitcoin; securing the blockchain comes as a secondary effect of that. Questions using this tag may be under the "why" and ask about the rationale for mining and behaviors that are observed in miners.
The "how" requires that a miner has specialized mining equipment. Questions using this tag may be under the "how" and ask about the exact process for mining, how mining equipment works, and how it secures the blockchain.