I'm no expert here. I may be corrected by someone who knows more than me. That said;
When things go across the net, they are in little fragments, called, you guessed it, packets. They vary in size. The packets are counted, especially for web pages and such, to insure that you got the whole thing. If it says there are 500 packets and you only have 450, it gets those other 50 or gives you a boo boo message.
Now maybe some other person can explain it better than me.