Few elements of a story can bring more depth and meaning than the play between the characters' inner and outer conflicts.
INNER CONFLICT takes place all in the character's head. It is usually some kind of fear or heartache or flaw in the character's belief system. Sometimes it can be helpful to reveal the backstory or reason behind this flaw. In fact, sometimes the whole story may be about that reason. But other times, all we need to know is that he has it, and wondering why has its own appeal.
OUTER CONFLICT is actually an odd way of defining what it is. Outer Conflict is basically the PHYSICAL CHALLENGE the hero needs to overcome. And in the story you're telling, this physical obstacle needs to have direct relevance to the character's Inner Conflict. (for example, the Inner Conflict could be that the character is afraid of the dark. And the Outer Conflict could be him having to go into a pitch black cave to rescue his dog).
While setting up your character, it can be very helpful to give him a distinct Inner Conflict before setting up the factors of his Outer Conflict, which is usually his main goal. Then once your character is on his adventure, he must first FACE AND OVERCOME his Inner Conflict in order to overcome the Outer Conflict, or physical challenge ahead of him.
It's worth noting that even though many books for beginning readers don't need to go into that much depth to be perfectly effective, many do. And if you can make a clear Inner Conflict a major part of your character, the chances are it will help.