Meaning of forklift tire 6.50x10-10pr

6.50 means that the tyre/tire is six and a half inches wide.

The 10 is the rim diameter in inches that it fits onto.

The 10pr stands for "ten ply rating" so the tire/tyre strength is equivalent to an old-time tyre that had ten actual plies of canvas in its construction. Nowadays steel and nylon are used as reinforcement and as far as I know canvas is no longer used. One problem with canvas or cloth reinforcement is that it eventually rots and the tyre bursts.