BBCode is used in the forum to format posts. For example choosing the font size and colour or making your links look better. There are several other uses for BBCode but those are the most used. But please remember that you do not have to understand or even use BBCode in order to post if you do not want to. You can still post your messages using nothing but plain text.
You can find some basic help how to use BBCode in your posts by clicking here. This help is also available in a link when you ar posting your messages. (You can find it under the smileys.)
Most BBCode can be automated by using the buttons above the post window. (When you are writing a message.) Just mark the text you want to manipulate and click on the wanted BBCode button.