The pleura lines the thoracic wall and diaphragm, where it is known as the parietal pleura. It is reflected onto the lung, where it is called the visceral pleura.
"Reflected" here describes the relationship between the continuous, but differently named parts of the pleura. The parietal pleura lines the chest wall and mediastinum (costal, cervical, mediastinal, and diaphragmatic areas marked in the image below). The visceral pleura covers the surface of the lungs (orange area below). There is a transition point at the root of the lung where the parietal pleura and visceral pleura meet. This transition is referred to typically as a reflection.
I think that the terminology is used to emphasize that the layers are continuous, but it is as if the parietal pleura bounces back off the body wall as visceral pleura.
Although not a particularly clear choice of words, because we usually connote "reflection" with light, in this case, "reflected" refers to how the parietal pleura folds back on itself from the inner chest walls and diaphragm to line the lungs.