In fiber-optic communication , a single-mode optical fiber (SMF) is an optical fiber designed to carry only a single mode of light - the transverse mode . Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining Maxwell’s equations and the boundary conditions. These modes define the way the wave travels through space, i.e. how the wave is distributed in space. Waves can have the same mode but have different frequencies. This is the case in single-mode fibers, where we can have waves with different frequencies, but of the same mode, which means that they are distributed in space the same way, and that gives us a single ray of light. Although the ray travels parallel to the length of the fiber, it is often called Transverse mode since its electromagnetic oscillations occur perpendicular (transverse) to the length of the fiber.

Source: https://en.wikipedia.org/wiki/Single-mode_optical_fiber