243 ways to win is a gambling term that refers to the mechanics present in video slot games and slot machines.
As you may know, every slot has reels and paylines. Reels are the horizontal columns that have symbols on them, and paylines are the vertical lines that go from one side to another. When it comes to paylines, they can be either straight or zig-zag lines. To land a winning combination, a player must land three or more symbols on an active payline.
Depending on the design of a slot, each game comes with a number of paylines. Some of them have as little as three paylines, while others have dozens and even thousands of paylines. The reason why they are important is that each payline represents a different way to win.
Therefore, 243 ways to win refers to the number of ways to land a winning combination. Coincidentally, it also tells you how many paylines a video slot has.