With this library you can transfer live network connections between two processes. I.e. you can update your service without lose the established TCP connections.
man 7 unix
SCM_RIGHTS
Send or receive a set of open file descriptors from another
process. The data portion contains an integer array of the file
descriptors.
Commonly, this operation is referred to as "passing a file de‐
scriptor" to another process. However, more accurately, what is
being passed is a reference to an open file description (see
open(2)), and in the receiving process it is likely that a dif‐
ferent file descriptor number will be used. Semantically, this
operation is equivalent to duplicating (dup(2)) a file descrip‐
tor into the file descriptor table of another process.