@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
@gaditb DIDs have a separate registry for all of the DID types that they maintain themselves, so you have to check the registry and look up wtf PLC is and how to resolve it
literally they just reinvented URNs but with a registry they own themselves. you can't resolve them without knowing what they mean