Time Portals are Magical gateways into the past and are major plot devices to the series. Jack has been seeking one since the beginning to undo the future that is Aku. The most notable ones are The Magic Wishing Well, The Lizard Monks', and The Guardians.
The origin of the Time Portals is unknown, though they may have been created by ancient Deities many years ago. It has appeared in many episodes.
In Episode XXXII: Jack and the Travelling Creatures, Jack fought against The Guardian to enter the portal, however, Jack was subsquently defeated by the Guardian. After Jack was defeated, The Guardian stated that Jack wasn't ready and the portal showed a vision of an older version with Jack with a huge beard and wearing a crown. It is unknown if this happens to be true.
Currently, all portals have been destroyed by Aku over the years, causing Jack to descend into intense rage and lose his balance.
Time Portals have varied in appearance, some having stylized gates, while some simply being a natural passage, though the majority share black and white rings in the portal itself.