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.
Amumu is good tank that has the ability to get many kills as well. This builds items make him very tanky with lots of armor and HP. Amumu is very dependent on his mana to get kills. The runes try to make up some lost ground in this category from the items.
High heath and armor with good mana and mana regen
low basic damage and attack speed
A little low on magic resist