The tertiary structure of a nucleic acid refers to the actual 3-dimensional shape that the molecule assumes. Recall that the primary structure simply refers to the ordering of bases along a strand of the acid, whereas the secondary structure refers to the way that bases from the same strand (RNA) or different strands (DNA) bond in pairs.
In the case of DNA, the tertiary structure is the famous double helix of Watson and Crick, a shape that can also be assumed by RNA molecules, which can take on a wide array of different tertiary structures, such as the one shown in the figure below.
