243 ways is the standard ways-to-win layout on a five-reel, three-row grid. Because each of the five reels shows three symbols, there are 3^5 = 243 different left-to-right paths, and matching symbols on adjacent reels from the left pay on any of them.
There are no lines to choose; every way is always active, and the stake covers all of them. Wins are usually priced per symbol combination, multiplied by the number of ways it lands on. Illustrative example: two cherries on reel one, one on reel two and one on reel three is 2 × 1 × 1 = two ways of three-of-a-kind.
The format is popular because it produces frequent hits on a familiar grid. It does not change the RTP; the paytable is set to fit. If reels expand to four rows during a feature, the count rises to 1,024.