|YOU'VE FORGOTTEN YOUR PURPOSE!
Time portals are magical gateways into the past or future, and were major plot devices throughout Samurai Jack. Samurai Jack sought one since the beginning of his quest to return to the past and destroy Aku once and for all so that The Future never come to pass. The most notable ones were the the Lizard Monks' Portal and The Guardian's Portal.
Most time portals that were seen in the series resembled a large circle with black and white rings alternately. The one that the Guardian guarded resembled a pillar of blue light and the time portal in Episode XIV: Jack Learns to Jump Good, was of blue and white circles. Some magical items such as the Crystal of Cagliostro, the Gems of Cronus, the Desert Oasis Jewel, Rope of Eons, and the Well of King Ozric were capable of opening a time portal. Some time portals may have only appeared at a specific time and then closed once and for all.
However, due to its initial travelling, It halted the aging process as Aku used it on Jack.
A non-magical time portal could possibly be created scientifically, such as faster than light space travel through a wormhole.
The origin of the time portals was unknown, though they may have been anomalies that were produced by the Earth. They appeared in some episodes.
All of the time portals that Aku was aware of were destroyed, which caused Jack to succumb to intense anger and frustration and cost him his sanity and his Sword, as he believed that he was doomed to wander the future forever (Episode XCVIII).
However, eventually, with the help of Ashi's powers of time travel, Jack traveled back to the past and destroyed Aku, which prevented the Earth and the universe from ever being ruled by Aku. It could also be presumed that the time portals were restored, and in some cases, ceased to exist (Episode CI).
The Guardian's Portal
Jack fought against the Guardian in order to enter the time portal. However, Jack was subsequently defeated by the Guardian. After Jack was defeated, the Guardian stated that Jack wasn't ready and the time portal showed a vision of an older version with Jack with a huge beard and a crown (Episode XXXII: Jack and the Travelling Creatures). However, Aku destroyed the Guardian's time portal and the Guardian was seemingly slain, although his remains, except what was left of his glasses at the base of the time portal, were nowhere to be seen (Episode C).
Unlike many others, the Guardian's time portal was only reserved for the use of the prophesied person that would defeat him. The time portal was also sentient, was capable of communicating to the Guardian telepathically, and told him to spare Jack as he was the prophesied one. It was unknown how Jack's alterations of the future affected the portal's existence since Jack was the only one who was meant to use it.
The Pool of Convergence
The pool of Convergence functions as a nexus of reality and is capable to function as a time portal into the past and future.