RE: Can someone explain...
A ventilated rib has lateral openings that will allow more air to circulate to increase the rate of cooling the barrel, and to minimize any distortion of the sight picture from heat waves.
A solid rib is a barrel/receiver rail that does not have any lateral openings in it. This is known as a box rib.
I found these on the MidwayUSA’s website. They have a Gun-Tec Library on their site that really helps. I hope these answers the questions you had.