Something that's one-way only allows movement in one direction. A one-way street is marked with an arrow, and you better follow it!
All cars, bikes, and motorcycles travel in the same direction on a one-way road, and you can say that traffic on one side of a divided highway is also one-way. You might encounter a one-way door at a supermarket, or a one-way line for entering a ride at an amusement park. You can also buy a one-way ticket on a bus or airplane, instead of a round trip that includes your ticket home.