The long-lived signature is an electronic signature that is valid over time.
This is achieved by including in it all the certificates in the chain of trust and the result of the validity check of the certificates at the time the signature was made. The time stamp is also included to identify the exact time that this long-lived signature was produced.
This re-signing process is used to ensure that data that was signed with an algorithm that was valid in its day (but insecure today due to technological evolution) does not lose value over time because secure cryptographic algorithms have been used at all times.