The Arch is constructed from several quantum power cells that have been linked together with the quantum energy the cells harness used to open an interdimensional portal.
In "Deals with Our Devils," AIDA constructed the Arch in order to return Coulson and Fitz, who had become trapped between dimensions, back to this dimension. Later, Robbie somehow activated the arch himself and used the portal to return to this dimension as well.
In "The Return," Robbie once again uses the Arch as a means of opening an interdimensional portal which he uses to return to this dimension.
In "World's End," after AIDA takes Simmons hostage, Fitz warns AIDA that Coulson has set a trap for her and plans on sending her to another dimension using the Arch with the Darkhold as bait. AIDA leaves to stop Coulson only to learn Fitz lied to her and their real plan is to have Ghost Rider, who is currently Coulson, kill her.