Ambulators were planeswalking devices first used by Phyrexians. Using one was an unpleasant experience for mortals, similar to being taken along by a planeswalker. When the Mending occurred, all existing planar travel tech, including all existing ambulators, stopped working.
On Urborg, Venser subconsciously built a machine that mimicked his latent abilities using parts from leftover Phyrexian artifacts. Later, a modified version was built with the help of the Ghitu. Since Teferi lost his spark when he healed the Shivan rift, the artifact proved to be invaluable.
In the end, Venser gave the ambulator to Jhoira, since he no longer needed it as a full-fledged planeswalker.