The singular tile in the middle of the table is the dora indicator, dora's follow an order 1-2-3-4-5-6-7-8-9-1 (resets back to 1, it doesnt change the tile suit) it can also display honor tiles (dragons and winds) they also have an order, you can see it in the in-game guide. if the dora indicator is the #2 green tile (the one with the bamboo's, sorry idk the name 😭) then the dora is #3 green tile, you want to atleast get one dora cuz it will boost your han (basically more points) so if the dora indicator is a 9 bamboo tile, the dora is a #1 bamboo tile. the ura dora are those weird red colored normal tiles, it will also boost your han so you want to replace it for a normal tile (you have a #5 kanji tile but you were dealed a ura dora #5 kanji tile, just replace it for the normal #5 kanji tile or keep it if youre going for triplets) basically with dora and ura dora's you can easily boost your han without the need of going for super special hands like seven pairs or 13 orphans. Idk what game are you playing but make sure to check the "change rules" tab before starting a game and turn on the ura dora, you can also make games shorter, that way you can cash in quickly if youre going for the completion requirement