FunctionType visit and binary search trees

In my textbook dealing with binary search trees, in the BST.h header file, they include a function thing as follows:

typedef void (*FunctionType)(TreeItemType& anItem);

They really don't give much of an explination as to what this does and I don't understand how you can write this syntactically or why it works. Also, each of the traverse functions such as

inorderTraverse(TreeNode *treePtr, FunctionType visit)

They use FunctionType visit as a paramater.

Also, there is another function that can be defined in the driver:

void display(TreeItemType& anItem); which is also used in some functions as a parameter of type FunctionType.

Can someone explain to me what is going on here because I just don't understand it. Thanks

Read this: http://www.function-pointer.org/
Then this.

gg