@gaditb did: is an IRI format which specifies a resolver (plc?) and a resource key; at:// is a protocol which takes a DID and indicates that it should be accessed using the AT protocol and then that the path should be followed (side-note: IETF does not like the name at: but has no mechanism by which to disallow it)
so this is saying
Access [PLC DID] with AT and then access the [app.bsky.feed.post] collection and then access item [ID] within it