Alvaro's portal is a teleporter which is found in the main chamber of the Salamandra base at the end of Chapter III, during The Unforgiven. It must be activated using a power stone to allow reinforcements from the witcher's allies to come through and aid him during the battle which takes place at the base.
Yaevinn mentioned that the portal was built by his kin, making it of elven origin, something which is also confirmed if Geralt choose to side with the Order. In that scenario, Siegfried blustered on about using such a device as it is clearly of evil elven origin.