Hi Daniel,
Representations of Uncertainty is one of the Tools & Techniques used in Quantitative Risk Analysis. It is basically the probability model you select to represent the unknowns (whether it be schedule, cost or resource requirements) for the quantitative risk analysis. (Pg 432 PMBOK 6)
For example, if this is a new project without previous lessons learned and not much reliable data available, I would choose a triangle distribution as my Representations of Uncertainty.
Hope this helps,
Joe