The points where the orbital planes intersect are called "nodes." At one node, Mercury is on the opposite side of the Sun from Earth. At the other node, Mercury is between the Sun and Earth. Sometimes Mercury can be seen in transit across the Sun. Transits take place only when both Earth and Mercury are on the same side of the Sun, and are both near the node at the same time. This doesn't happen very often. Transits of Mercury occur only about 13 times a century.
