They are markets that produce alterations (waves) in companies in a constant and non-linear way but in an exponential way.
In this type of market, more traditional business management practices or agile frameworks don't usually work properly, except in the initial stages of exponentiality, when the exponential growth is flat and similar to linear growth.
Exponential markets are unpredictable, but some patterns can be relied upon to find a better option for a given time.
For more detailed information about them, check Enterprise Agility Fundamentals.