What does the red symbol mean here?
If you didn't cut off the picture, you'd see the wires are labelled AIN15_P, AIN15_N, AIN16_P, AIN16_N. The red symbol tells Altium that these signals (each matching P and N pair) should be considered a differential pair. 

This can allow the layout tool to invoke special design rules for these nets, and the autorouter or interactive router to treat them specially.