Why don't you post the context as I suggested - we have no idea which processor you might be using - if you would post a link to the document that you don't undertsand we could helo you properly instead of guessing.
A number of computer architectures use a https://en.wikipedia.org/wiki/Link_register to store the return address for a subroutine call. The instruction set typically has a "Branch And Link" instruction that jumps to an address and simultaneously stores the return address in a link register. For example, the IBM 360/370 has a "BAL" instruction. ARM and PowerPC have a "BL" instruction. There are also versions of these instructions for when the target address is in a register, such as "BALR" and "BLR".
Some architectures have a single dedicated link register (LR). Others allow any register to be used, but one is usually used by convention.
