In this model, two transmitters broadcast signals that may collide with one another. The receiver will correctly receive a signal if the signal to interference ratio is 3dB or better, meaning that the received signal has at least twice the power of the interfering signal. When Transmitter1 is far away from the receiver, it cannot interfere with the transmission of Transmitter0. However, when it gets close, it can.
The upper plot shows the received power density (in watts per square meter) seen by Receiver for both transmitters. It also shows the duration of the transmissions. If the transmissions overlap, then one of the transmissions is surely lost. The second is lost only if its power is not greater than twice the power of the interferer.
The lower plot shows which messages were received and which were lost. The messages from Transmitter1 will always be lost if they collide with messages from Transmitter0. However, messages from Transmitter0 can only be lost when Transmitter1 is close to the Receiver.