Mining is the action of using a worker unit to gather minerals. Protoss has the best mining rate out of all the races due to the unit collision shape of the Nexus. On the other hand, Terran is the slowest miner out of all the races due to the smaller collision hitbox of the Command Center. Contrary to popular belief, all workers move and accelerate at the same speed -- it is only due to the shape of the resource gathering center that mining rate differs between races.
The relative mining efficiency of workers are as follows: SCVs mining at 100% efficiency, drones at 182/174 = 105% efficiency and probes at 182/154 = 118% efficiency, based on the mining animation lengths.