A high profile vehicle is a vehicle such as a semi truck, pickup truck, Van or SUV.
High Profile vehicles are basically any vehicles with large surface areas that can easily be exposed to high cross winds and are more prone to being blown over by the strong winds.
Low profile vehicles are those that sit lower to the ground and high profile vehicles sit higher off the ground.
You usually hear the warning about high profile vehicles when a high wind warning is occurring or is expected to occur.