Taking Bitcoin as an example, in terms of POW (Proof of Work) as the currency of the consensus protocol, mining is the process of mining machines to solve the "problems" published in the Bitcoin network and to win the reward.
For example, the Bitcoin network lists a problem at some point: X+1=2, solve for X?
If one solves X=1 first and broadcasts it among all the mining machines connected to the Bitcoin network, that means this mining machine succeeds in mining and gets the Bitcoin reward from the Bitcoin network.
This is the basic principle of POW (Proof of Work) mining. When the mining machine solves X, it will prove that it has completed a certain amount of work so as to get rewards. The process of the mining machine to solve a complicated mathematical puzzle to get rewards is called mining.