A map ratio or scale factor tells you how much each unit of distance on a map represents in reality. For example:
1:50,000
means 1 cm on the map represents 50,000 cm in real life.
There are 100,000 cm in 1 km so we can think of this as 1 cm representing 0.5 km or as 2 cm representing 1 km.
Map ratios or scale factors are normally written in the form 1:? so that a reader can see exactly what 1 cm represents on the map.