It means exactly as it says... how far is the position, in bytes, of an item currently being referenced, from the current RIP register (after the next instruction because RIP points to the next instruction). For example;
0x400100 call myTestFunction ;by this time RIP points to 0x400107 (next instruction)
0x400107 nop
0x400108 nop
0x400109 myTestFunction:
...
ret
Therefore the encoding for call myTestFunction should be E8 02 00 00 00 because it is located two bytes off the next instruction (the first nop). This is just a pseudo example.