This is described in theory of plates. Mx, My and Mxy are resultant of stresses. For a rectangular plate element, the stresses of Mx and My are normal to the orthogonal planes. Those of Mxy are tangential. How to well make use of them? I think the more simple answer is, through the stresses they represent. In any case there are examples in the books and program manuals dealing in one way or another account this. RISA 3D has explanations about in the manual.

Yes, agree with ishvaaag.  Just an simple example to let you visualize the three:

Take a sheet of square paper in your hands.  If you bend the paper such that it looks like a "U" shaped half barrel; you would be applying a moment to the paper much like Mx.  My is simply the same think only perpendicular to Mx.

Now Mxy would be represented by you taking your left hand and rotating the left edge of the paper forward and away from you.  Take your right hand and rotate the paper up and toward you.  In other words, one side of the paper is rotating clockwise and the opposite side of the paper is rotating counter-clockwise.  This is a kind of twisting moment that is also represented on the near and far edges of the paper at the same time.

What you have to do with your finite element results is ADD the Mx with the Mxy at any one point to get the full bending of the elements at that point.  Also, you ADD the My moment to the Mxy moment as well.  Sometimes getting the actual Mxy and My or Mx to add across a surface is difficult.  What we sometimes do is go conservative and just add the largest Mxy to any Mx or My to get a design moment.  Most times the Mxy isn't real big compared to the other two moments.

Look up the Wood-Armer method which takes Mx,My and Mxy and converts to figures that can be used for reinforcement design (skew or otherwise).

where can i find information about the Wood-Armer method?

TO ERNESTOBARRERA
You can go to the following site and you can find a spreadsheet
about WoodArmer method :
http://www.childs-ceng.demon.co.uk/