This is a tool that can be used to calculate the absolute target address of a "rjmp" relative jump instruction on AVR microcontrollers.
AVR microcontroller's use the "rjmp" instruction to 'jump' to a new location in memory, relative to the current location. The primary advantage of the rjmp instruction is that it is smaller, so it takes less of your precious program space. rjmp is specified as 12-bit signed offset to the word address from the next instruction after the rjmp itself.
The rjmp at address 10E jumps to address 10C, or backwards 2 words from the next instruction after the rjmp.
This tool allows you to enter the two byte values found in AVR flash memory, and it reports the effective jump address. To calculate the absolute address from a relative jump, it also needs to know the address of the rjmp instruction itself.