Quick binary tree description: each node has two children, the left one is smaller, the right one is larger. But the right one or both may be empty, meaning no more children.
To walk a tree in order
1 start at the root
2 go left as far as you can
3 back up to the last place you could have gone right
4 go right
5 repeat from 2
Draw some on paper and try it:
See if you can write a Node class with a value and two references to child nodes, load up some values and walk them in order. Show us your work! That's how we know where you're stuck.