"Transonic" is used to designate a range of speeds comprised between Mcrit & Mach 1.
Mcrit (critical Mach No) is the lowest speed @ which compressibility effects start to act, producing a large increase in Drag & a decrease in Lift (& therefore a large decrease in the L/D ratio). Mcrit is increased (enabling the aircraft to reach higher speeds before being subject to compressibility) when the aircraft is fitted with swept wings.
Even if the aircraft is not flown @ or above Mcrit, this speed can actually be exceeded @ some specific points of the airframe. What U heard therefore means that, due to their own movement, the blades will, @ some point, actually reach a speed just above Mcrit, whilst the aircraft's overall speed will stay below it.
This condition is not ususual for turbine blades in jet aircrafts, but I've never heard it applied to prop blades before. So there might be another specific meaning, but the above is all I could think of.
MF