Rekenrek - you might think of an abacus or arithmetic rack which would be pretty close to exactly what it is. This tool has 2 to 10 rows of five red and five white beads. This math manipulative can be used to develop number sense in the elementary classroom - usually from grade K-3.
These tools can be purchased or even a version of them can be made (I suggest small group) by your students to make sense of the tool.
There are so many examples and resources for your use.
This blog contains a free set of questions to use with your class!
http://www.k-5mathteachingresources.com/Rekenrek.htmlVirtual Rekenrek from Didax