These polystyrene mats are to support the underside of a glass aquarium.  They even out any imperfections on the cabinet or stand, but you should still make sure it's a smooth, stable and level surface!  They come in standard 12" x 12" (30cm x 30cm) size and can easily be cut with scissors to non-standard sizes.  For larger aquariums, buy several and line them up to support the whole base.