Don't worry about your English, there's plenty of non-native speakers here
Packets are bits of data sent through a network using UDP or TCP. In java you can send many primitives through the built in Java sockets, ad I suggest you learn the built in Java methods first though because it's nice to know them. Then check out Kryonet as ctomni suggested.
Those links quew8 gave you are a great resource too.
There's a great thread that's pretty much stickied on this board labeled UDP vs. TCP/IP, you should check that out too.