Toa Metru was the name given to the six Toa heroes who defended Metru Nui against Makuta, the Morbuzakh, and the Dark Hunters. The Toa Metru were Vakama, Onewa, Whenua, Matau, Nuju, and Nokama. It was believed that these six were the first Toa to take this name, as the previous Toa serving in Metru Nui were not native to the city.
In the time before time, six Matoran were mysteriously gifted with the powers of Toa. Their destiny was to protect the city of legends, Metru Nui, from all evil. These Toa Metru had to master their elemental energies and Toa tools, uncover the mysteries that threatened their home, and learn to be heroes.
The Toa Metru fulfilled their destiny by saving the Matoran from Makuta.
The Toa Metru had the potential to form Toa Kaita, but they never did so because they simply did not know how to accomplish the feat.
The Toa Metru were Toa for a few months.
The Toa Metru were the first Matoran from Metru Nui to become Toa.
The Toa Metru did not know how to use their elemental powers well at first, so they tended to use too much too fast and run out. They also had to be more careful about how they used them. They were in a city, not a wilderness, and creating tidal waves, blizzards, etc. would do a lot of damage to the city, so they had to be more subtle about how they used their powers.