See an explanation on Wikipedia and on MSDN.

Additional notes:

• In an x86 environment, the FS register points to the current value of the “Thread Information Block” (TIB) structure. (ref.),(ref.)
  • Wikipedia on TIB (Thread Information Block) / TEB (Thread Environment Block)
    • specifically, FS:0x2C - “Linear address of the thread-local storage array” – see also: get tls

• get tls
• M, G