Hi there!
This is a great question, and one that I actually see really often.
A defining clause is information given to provide specifics about the word it modifies (often a noun). It helps the reader (or listener) know about which specific subject you're talking.
So, for example:
The woman who stood near the tree was crying.
In this case, the defining clause is who stood near the tree. This lets your reader/listener know that if there are multiple women, the one to which you're referring is the one standing by the tree.
Here's another example:
The bell pepper that I bought at the store is moldy.
One final tip-- don't offset defining clauses with a common. It's not necessary and can change the meaning of the sentence.