Refers to a wind that blows in the direction of travel, making sailing or progress easier. Also used figuratively for favorable circumstances.
With a favorable wind, the ship sailed smoothly.
The new business got off to a very smooth start.
smooth sailing; everything going well
smooth sailing; plain sailing; everything going well
Compound of 順 (jun, 'following, favorable') and 風 (fū, 'wind'). The figurative sense arises naturally from the idea of a wind that aids progress.