This is engine talk. Inside an engine there is a piston that slides up and down inside a tube, and each single up or down movement is called a stroke. A two-stroke engine finishes its whole cycle — pull in fuel, burn it, push out the smoke — in just two of those movements, one up and one down. A four-stroke engine takes four. Because a two-stroke fires every single time round, it gives a lot of power for its size and weight, and it is simpler and cheaper to build; that is why you find them in scooters, small motorbikes, chainsaws, lawnmowers and outboard boat motors. The trade-off is that they burn oil mixed into the fuel, so they smoke, smell, and pollute badly — that high buzzing sound and the blue smoke from an old scooter is a two-stroke. This matters in Taiwan: the government spent years paying people to scrap old two-stroke scooters (二行程機車) because of air pollution, so you may know the Chinese word from the news even if the English is new. Use it before a noun — 'a two-stroke engine', 'a two-stroke scooter' — or on its own as a noun for the engine itself: 'it's a two-stroke'.