The term stepping level in the context of CPU architecture or integrated circuitry is a version number.
Stepping level refers to the introduction or revision of the lithographic mask or masks within the set of plates that generate the pattern that produces the CPU or integrated circuit. The term derives from the name of the equipment ("steppers") that exposes the photoresist to light.[1]
Typically, when an integrated circuit manufacturer such as Intel or AMD invests money to do a stepping (i.e. a revision to the masks), they have found bugs in the logic, have made improvements to the design that allow for faster processing, or have found a way to increase yield or improve the "bin splits" (i.e. create faster transistors and hence faster CPUs). One result of some new steppings is that the CPU design is improved such that it overclocks better than others.[1]
Many CPUs have a means of interrogating them in order to discover their stepping level. For example, on x86 CPUs executing the CPUID with the EAX register set to '1' will place values in other registers that show the CPU's stepping level.