It is Ramsdell’s notation for stacking of hexagonal layers. The number shows how many layers in a unit cell, and the following letter indicate lattice type, could be hexgonal(H), rhombohedral(R)... For example, 2H is typical HCP structure with stacking of ...AB....
Ramsdell’s notation is simple but in most cases you could not tell about the real crystal structure, since at least the displacement between the layers is not specified.