Can someone explain what 50/50 or 60/40 means when it comes to shotguns?

50/50 means the gun shoots flat. On a patterning board if you center the shot, the pellets of the pattern will be distributed approx. 50% above the middle line and 50% below the middle line. A gun that shoots flat has the middle beat and the front beat on the same line, without any visible gap between them. Skeet shooters are most likely to shoot a 50/50 shotgun considering the flatness of the target flying path.

In the 60/40 configuration the shot pattern is distributed 60% above the middle line of the patterning board and 40% beneath. The two beads form figure 8, with the front bead being above the middle one. This setup is used in Sporting Clays where the shooter encounters a vast variety of target presentations. One thing to note, the gun has to be inserted below the target path to compensate for the high setup of the gun.