SigHax(often called B9S, because of Boot9Strap, an implementation of this) is an exploit taking advantage of a flaw in the ARM9 bootROM of the 3DS, causing the signatures (which those of you less technically oriented may think of as "proofs of authenticity" that normally only Nintendo can generate) for arbitrary firmwares to be read as valid. On a normal boot, if one modifies the header for the firmware partition stored in NAND, the signature's proof of authenticity will fail to validate, and the firmware will be rejected. SigHax allows us to make every modified firmware header read as valid, and thus allows for loading custom code from the NAND's firmware partitions.
. This exploit is often used to boot CFW at boot time, which in turn patches signature checks and blocks FIRM writes (as to keep the exploit intact even after a system update on SysNAND).
At GS2012 Mirror, multiple builds of SigHax and its variations are available.
You can check these below.
WARNING: Do not mess with this if you don't know what this is, or you could end up with a brick.
Boot9Strap - This is the most used SigHax implementation.