The mass line is a way of ensuring a dialectical relationship between the vanguard party and the masses. The masses educate the party through consultation on their needs, the party educates the masses to develop their consciousness. The party trains and guides the most developed and revolutionary members of the masses, and the masses supplies those members to the party. It’s a strategy to prevent the worker’s party becoming detached and losing sight of the needs and goals of the proletariat. It was one of Mao’s strategies to prevent the descent into revisionism he saw in the USSR.

In practice it looks like surveying people about their needs - maybe some town has poor water supply. The party takes this complaint, draws up plans and so on. These plans would then also be taken for discussion with the masses. It’s an ongoing thing. Even once you say, built your new water system, the masses would be consulted again, to see whether it was effective, whether more was needed, what was good and bad about the project, etc.

As communists we believe the conscious/revolutionary proletariat to be the most developed social force in history, the mass line is a way of harnessing that force rather than excluding it from political rule.