Bitcoin core config explanation
-dbcache: bitcoin is doing heavy disk I/O, you can reduce disk I/Os with increasing this parameter and thus increase memory usage

-par: if your system is so slow, you can't use it anymore, limit the number of threads or CPU/cores that are used

-pid: this is usually the process ID on your unixoide system. if you start a single instance, there is nothing to play with...

-bind: if you have several network cards in your system, you can bind the process to one of the many IP addresses in your system

-discover: discover IP addresses in the network. Set to 1, else set to 0, e.g. if you are using connections over TOR. 

-upgradewallet: - someone else please :-)

-rpcbind: if your system is multihomed, and you want to talk from several networks to your machine, you can tell the bitcoin daemon, to listen to only a specific address for remote procedure calls

-rpcthreads: 


  how many connections can a thread afford?  


good question, I don't know. Experts please. I was of the opinion, that one thread can handle one connection... 


  and how the connections counted?


not sure what you mean by this - if you have 4 clients, that connect to your system, then you have 4 connections. Or 5... or 6 ... and you can have 4 different systems talk to your bitcoind.


  is it per IP address or command?


per IP address.

I know my answer is not 100%, so I am asking for additional help as well.