Master Yi Build
Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
The reason you should do a 23/0/7 mastery tree is because you are a fighter. There is no need for you do do any defense, your main attribute should be damage. The rest are put into utility because there's no point in being divided between dps and tank, the utilities you put in will help you lane longer and farm more before you run out of mana.
AS = Alpha Strike
M = Meditate
WS = Wuju Style
Ult = Highlander/Ultimate
Early Game: Try not to die, do some harass/farm with AS
Mid Game: This is when you'll start dealing massive damage. Start ganking and doing some team fights.
Late Game: With your bloodthirster stacks, you can no longer die. FINISH IT