What does the + sign mean in reference to An aircraft's Mach speed?

It could be a number of reasons. If you see that on the specs for a military plane, it usually means the actual top speed is clified and the number given is the only one the military will publicly admit. Also, the actual top speed varies depending on conditions (altitude, temperature, aircraft weight, external stores, etc), so to make things easier, they might just pick the top speed for a typical mission and configuration, even though in other situations the aircraft might be capable of going slightly faster. The last possibility is that the actual top speed is only slightly above the listed speed (for example it might be Mach 2.57, but rather than rounding up, they'll say 2.5+ instead).