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.
Kog'Maw is a champion where you go in deal a lot of damage and try to get out. If you don't it doesn't matter since his passive allows you to run around for 4 seconds then explode dealing a lot of damage. Kog'Maw is a very suicidal champion. This build allows you to get in and do tons of damage and the hopefully get out due to very high speed.
Extremly high damage
Good speed that allows you to get away
High mana reen allowing you to use his ultimate very often
Fairly low health means you die often