>> is arithmetic shift right, >>> is logical shift right.
In an arithmetic shift, the symbol bit is stretched to save the signedness of the number.
For example -2 described in 8 bits would be 11111110 (because the most important bit has negative weight). Changing it right one-bit applying arithmetic shift would give you 11111111, or -1. Logical right shift, though, doesn't bother that the value could serve a signed number; it just moves everything to the right and fills in from the left with 0s. Changing our -2 right one bit applying logical shift would give 01111111.