This is the server checking to see if you are there. If this response time check from the server is too long the server will disconnect you. It's called Ping Timeout. There are many servers to choose from. The server is a computer somewhere that you connect to, and in turn is connected to the Intrenet and other IRC servers.
Less splits occur when everyone in a channel is using the same server.
When you log onto a server there is a net connection between you and your server. When you are chatting with someone else they also have a net connection between them and their server. The two servers are also connected through the net. A net split is when a connection along the path loses it's connection. You become disconnected from the others you are talking to until the connections are re-established.