PEX-AL-PEX pipe, or PEX/Aluminum/PEX, are made of a layer of aluminum sandwiched between two layers of PEX. The metal layer serves as an oxygen barrier, stopping the oxygen diffusion through the polymer matrix, so it cannot dissolve into the water in the tube and corrode the metal components of the system. It also bends better, because the shape memory of the PEX is offset by the aluminum.
One alternative to PEX-AL-PEX is HE-PEX. Both products have an oxygen barrier, however HE-PEX does not have the same shape memory feature of PEX-AL-PEX. HE-PEX is (also) manufactured by Uponor.