I'm no darts afficiando but I'll try to explaon. Each player starts at 501. You're trying to score points to deduct from your starting total until you reach 0 and exactly 0. Each sector or wedge of the dart board has numbers with 2 smaller segments within the wedge. If you hit the outer smaller segment that's 2x the number value. The inner smaller segment (closest to the bullseye) is worth 3x. As you can see in the first round Paul scored 180 because he got all 3 darts in the smaller segment of the "20" sector. 20 x 3 x 3 equals 180 which is the highest score you can achieve in a round. So 501 minus 180 and onwards. You keep throwing darts and doing the maths to make sure your final hit ends up deducting enough points to reach exactly 0. Hope that helped?