Ways to win replaces fixed paylines with a rule: any matching symbols on adjacent reels, starting from the leftmost, form a win, whatever row they are in. The number of ways is the number of symbol positions on each reel multiplied together.
On a 5x3 grid that is 3 × 3 × 3 × 3 × 3 = 243 ways. On a 5x4 grid it is 1,024 ways. If the same symbol appears twice on one reel, each copy counts, so one result can pay several ways at once.
More ways does not mean a better game. The paytable values are scaled to match, so a 243-ways game is not more generous than a 20-line one. The difference is in feel: ways games tend to produce frequent small wins, many of them below the stake.
Games that change the symbols per reel on each spin, such as Megaways, have a variable number of ways. See the ways to win feature page for worked examples.