RAPL is short for Running Average Power Limit and since Sandy Bridge there has been this RAPL interface for exposing power meters and power limits. This power information is exposed through MSRs and the PCI Express config space, which in turn are exposed under Linux using this RAPL kernel code.
RAPL exposes energy counters and performance counters that Intel believes matches actual power measurements. RAPL also allows for setting power limits on the processor and DRAM. RAPL also exposes performance feedback including the time the RAPL mechanism forced the P-State below the OS-requested P-State and other metrics. Among the software taking advantage of RAPL on Linux is TurboStat, PowerTOP, and the Linux thermal daemon. 01.org.