Ply means to work steadily at something. If you've got a lemonade stand and you're busy perfecting your recipe and seeking out customers, you're doing a good job of plying your trade.
A ship that regularly travels a particular route plies that route. When someone offers you food or drink, often in hopes of getting something in return, they're plying you: "If I ply you with cake, will you take my dog for a walk?" The noun ply is very different from the verb, referring to layers or strands. Four-ply yarn is made up of four strands. Plywood is made of many layers of wood glued together.