I'm reading the address management of Bitcoin core implementation. There is one thing I couldn't understand is in which condition an IP address in the new table of a node is overwritten?

In particular, when the node hears about a new peer, it adds that IP address into a slot in the new table, if that slot is occupied, it checks if the existing isTerrible: https://github.com/bitcoin/bitcoin/blob/master/src/addrman.cpp#L309

if (infoExisting.IsTerrible() || (infoExisting.nRefCount > 1 && pinfo->nRefCount == 0))

What is nRefCount? What is the idea behind this check?