What do fractions mean when talking about electrical wiring
It is not a fraction. 18/5 means that there are five (5) conductors and each is 18 AWG (american wire gauge). The gauge relates to the current carrying capacity of each conductor.

It was all specified in the title of the amazon link that you provided:

Southwire 64169622 5 Conductor 18/5 Thermostat Wire, 18-Gauge Solid Copper Class 2 Power-Limited Circuit Cable, 50-Feet, Brown