"Stepping" refers to the version/revision number of a given chip.
What happens is over the lifetime of the thing, the manufacturer can make changes and improvements to the same design. Things like updates in the microcode, or adjustments to manufacturing, bug fixes, etc... Depending on the nature of the change (and whim of the manifacturer) the Stepping could receive a 'minor' update: B1 to B2, etc... Or a major update: B2 to C1, for example.
To the great majority of the computer using public, what stepping a chip may be is of zero concern. Overclockers and enthusiasts do pay attention to stepping, since there can be improvements in a given design's performance and "Overclockability".