CRA is an approach used to help students learn and build upon new mathematical concepts.  In the concrete stage the student is manipulating objects to solve a problem.  The concrete stage is the "doing" stage.  In the representational stage the student is "seeing" the problem and drawing a representation or picture of it. In the abstract stage the student is able to use symbols to represent the parts of the problem and is considered the symbolic stage.  For more information on the CRA approach to teaching mathematics please see the article at http://cbennettrivier.wordpress.com/2012/03/29/concrete-representational-abstract-instructional-approach/.