Players in your defence are called defenders. They are the ones that prevent the opponent from scoring goals. Defenders are the most deciding factor in your defence ratings.
There are three parts of the defence Left Defence, Right Defence and Central Defence
This means that having skilled defenders and/or many defenders (i.e. playing 5-3-2 or 5-4-1) will boost your defence ratings and thus reduce the chance of your opponent chances to score a goal.