This is highly simplified:

Your machine creates a message and chops it into tiny packages known as
"packets" to send across the internet.
 
Each packet is tagged so that, when it reaches a server, the server can
read use the tag to put the packages back into the right order and read
the message you sent it.
 
If there is a problem between the two systems - some packets get lost
and the server has to ask for them again. If this happens enough then
you start to see lag/latency.
 
See help for mtr for how to see if this is what is happening and where.