The Matoran put on a show to celebrate the Toa's defeat of Makuta (see Mata Nui Online Game).
Then, naturally, disaster strikes in the form of Bohrok, and as the hero, you're expected to take care of it.
Tahu running around.
Gather the Lightstones.
Tahu wears the Kanohi Hau, the Great Mask of Shielding, which allows him to create a shield around himself to protect him from attacks. It doesn't always help.