When you squish through a muddy field, your feet make sucking, sloppy sounds as they move. If your sneakers get wet in the rain, you'll have to squish down the street.
The verb squish is perfect for describing a walk through any muddy or wet situation. People squish through puddles and in soggy shoes. You can also refer to the sound itself as a squish: "The squish of your boots in the hall told me it must have started raining again." Squish is imitative — in other words, it sounds exactly like its meaning. It's thought to be a variation on squash and influenced by the obsolete word squiss, "to squeeze or crush."