Java is a programming language that some of the running software on your pc is using, so it probably gets activated when something else starts, then checks the net to see if it's up-to-date, and if there is a newer version you get told about it in the system tray near the clock in the bottom right hand corner of your screen.
As long as it's not a really old computer, you should have a pretty up-to-date version of Java anyway, so you can just turn off Java's automatic update feature....