It means that when your node received that block, the block was more than 30 minutes old already (its timestamp was from more than 30 minutes ago). Your endorser correctly acknowledged that the node just received an "endorsable" block (a block on which you have endorsement rights) but concluded that it was too late to forge the endorsement.
Rationale: Endorsements for block
n can only be included in block
n+1. In 30 minutes, there must have been a successor block published already, it's too late for an endorsement published now to be part of it.
I see 2 reason for that to occur:
- your node is very late. Try to understand why you received the block only now.
- your machine is not on time at all. The clock on your machine lives in the futur, resynchronise it.