What does reduced level mean?

In the context of Civil Engineering, especially land surveying, Reduced Level "RL" refers to reducing (or equating) levels (elevations) to a common datum, which is either a real or imaginary location with a nominated elevation of zero.

The most common convenient datum was mean sea level.

On small surveys that require the collection of elevation data the datum can be assumed and for practical purposes it simply needs to be far enough below the survey area to avoid negative numbers, thus the starting point of a survey might be nominated to be RL 100.00