To swish is to make something rush or hiss as it moves, or to move this way yourself. A bike might swish past you on a wet sidewalk, and you might swish your long dress as you continue walking.
A swish is the sound that's made when a person or thing swishes — the swish of a dish brush on a dirty pan in the sink, or the swish of water against a dock. A kite swishes through the air and so does a fencing epee or a fly swatter. In basketball, you swish when you make a clean basket, without the ball hitting the rim or the backboard. Swish is an example of onomatopoeia — when a word that sounds like its meaning.