A HIT means that the document was found in the cache. A MISS, that it wasn't found in the cache. A negative hit means that it was found in the cache, but it doesn't exist. The hostname is the name that was requested to be resolved. N Means negatively cached.
P Means the request is pending being dispatched.
D Means the request has been dispatched and we're waiting for an answer. L Means it is a locked entry because it represents a parent or sibling. The N column is the number of IP addresses from which the cache has documents. The rest of the line lists all the IP addresses that have been associated with that IP cache entry. This question was asked on the squid-users mailing list, to which there were three excellent replies.