From Microsoft's article "Blue Screen Data":
The hexadecimal number following the word "STOP" is called the bug check code or Stop code. This is the most important item on the screen.
Each bug check code has four associated parameters. In the first blue screen shown here, all four parameters are displayed after the bug check code. However, in the second kind of blue screen, these parameters have been rearranged within the explanatory text. Regardless of the amount of rearrangement, they will always appear sequentially. If fewer than four parameters appear, the remaining parameters can be assumed to be zero.
What those parameters contain/represent depends on the Stop code, as well as the other parameters.
Wikipedia puts it nicely:
"Depending on the error number and its nature, all, some, or even none of the parameters contain data pertaining to what went wrong, and/or where it happened."
For the bug check code 0x124 (WHEA_UNCORRECTABLE_ERROR), for example:
╔═════════════╦════════════════════════════════════════╦════════════════════════════════════════════════════════════════════╦═══════════════════════════════════════════════════════════════════╦════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╗ ║ Parameter 1 ║ Parameter 2 ║ Parameter 3 ║ Parameter 4 ║ Cause of error ║ ╠═════════════╬════════════════════════════════════════╬════════════════════════════════════════════════════════════════════╬═══════════════════════════════════════════════════════════════════╬════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╣ ║ 0x0 ║ Address of WHEA_ERROR_RECORD structure ║ High 32 bits of MCi_STATUS MSR for the MCA bank that had the error ║ Low 32 bits of MCi_STATUS MSR for the MCA bank that had the error ║ "A machine check exception occurred. These parameter descriptions apply if the processor is based on the x64 architecture, or the x86 architecture that has the MCA feature available (for example, Intel Pentium Pro, Pentium IV, or Xeon)." ║ ║ 0x1 ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ A corrected machine check exception occurred. ║ ║ 0x2 ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ A corrected platform error occurred. ║ ║ 0x3 ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ A nonmaskable Interrupt (NMI) error occurred. ║ ║ 0x4 ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ An uncorrectable PCI Express error occurred. ║ ║ 0x5 ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ A generic hardware error occurred. ║ ║ 0x6 ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ An initialization error occurred. ║ ║ 0x7 ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ A BOOT error occurred. ║ ║ 0x8 ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ A Scalable Coherent Interface (SCI) generic error occurred. ║ ║ 0x9 ║ Address of WHEA_ERROR_RECORD structure ║ "Length, in bytes, of the SAL log." ║ Address of the SAL log ║ An uncorrectable Itanium-based machine check abort error occurred. ║ ║ 0xA ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ A corrected Itanium-based machine check error occurred. ║ ║ 0xB ║ Address of WHEA_ERROR_RECORD structure ║ Reserved ║ Reserved ║ A corrected Itanium platform error occurred ║ ╚═════════════╩════════════════════════════════════════╩════════════════════════════════════════════════════════════════════╩═══════════════════════════════════════════════════════════════════╩════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════╝
For more information on how to use these parameters to actually debug it, check out this other SU question: How to diagnose Blue Screens in Windows 7 (64bit)