Multimode fibre cable has multiple modes of propagation—that is, several wavelengths of light are normally used in the fibre core. In contrast, single-mode fibre cable has only one mode of propagation: a single wavelength of light in the fibre core. This means there’s no interference or overlap between the different wavelengths of light to garble your data over long distances like there is with multimode cable.

What does this get you? Distance–up to 50 times more distance than multimode fibre cable. You can also get higher bandwidth. You can use a pair of single-mode fibre strands full-duplex for up to twice the throughput of multimode fibre cable. The actual speed and distance you get will vary with the devices used with the single-mode fibre.