Toa Canisters were transports in the shape of a pod that carried Toa from one island to another across the Great Sea. These canisters were crafted by an unnamed Av-Matoran in Karzahni. These pods had beacons that would respond to the activation of Toa Stones. Toa Canisters could be used by other beings once they were discarded, such as the Piraka.
The Toa Mata, after completing their training, were placed in Toa Canisters, which were then placed in special niches in the Codrex. They were later summoned to Mata Nui when Takua placed six Toa Stones into a Suva.
The plastic canisters that contained the Toa Mata and Toa Nuva sets could be used as Toa Canisters for the Toa that they came with.
When the Toa Mata first emerged from their canisters on Mata Nui, they were in pieces, and had to reassemble themselves.
Normally no one with less than the power of a Toa is able survive the use of a Toa Canister.
The makers of the Pods used an odd form of logic in the construction. Namely that they were made in groups of six, because the seventh Toa would come in the dark times. No seventh Canister, no seventh Toa, no dark times.
Although it is commonly believed that Tahu was the first of the Toa Mata to arrive on Mata Nui (as he's the team leader), Onua was actually the first Toa to reach the island. Tahu was second.