The value will include all incoming connections including any shell connections or connections from other servers, such as replica set members or mongos instances.
连接,可用
The number of unused incoming connections available. Consider this value in combination with the value of connections.current to understand the connection load on the database, and the UNIX ulimit Settings document for more information about system thresholds on available connections.
This shows that I currently have five connections open to the MongoDB port (27017) on my computer. In my case I'm connecting to MongoDB from a Scalatra server, and I'm using the MongoDB Casbah driver, but you'll see the same lsof TCP connections regardless of the client used (as long as they're connecting using TCP/IP).