Under the old rubrics a curtain inside was required, as well as a veil which was supposed to cover the entire tabernacle, not just a little curtain on the front that you would sometimes see. It was required that it be the same color as the vestments of the feast that day, except for black (if there was a funeral) in which case it and the antipendium (also not always used even when it was still required) could be purple. The veil could always be white, if it was not changed on a daily basis. Since the tabernacle was a large tent during the Exodus, the veil reminds us of the tabernacle in the desert according to the older rubrics. The veil inside the tabernacle doors like the veil in the holy of holies.
Most people look for the sanctuary lamp, an oil or wax candle to signify the reservation of our Lord in the tabernacle, however in the past it was the tabernacle veil which signified to Catholics that our Lord was waiting for us in the tabernacle.