Code 0 - Network Unreachable
Either the network is closed off and not accepting communication or there's no next hop available
Code 1 - Host Unreachable
Either the end host is off or the remote gateway failed to find the destination despite the request reaching the network
Code 2 - Port unreachable
Either the router/device isnt accepting incoming traffic or in the instance of a tracert, an end host has been reached, or the device was unable to process the request.
Code 3 - Fragmentation needed and DF was set
A packet was sent to a router that was too big for that router to handle. The router would like to fragment the packet, break it down to smaller pieces, but the packet is set to DF or "dont fragment"