Teaching and learning to decompose numbers with the use of number bonds is one of the essential Singapore Math strategies in the curriculum. Put simply, a number bond is a graphic representation of the "part-part-whole" relationship between 3 numbers. Here is a rudimentary example:
Note the larger circle for the whole, and the smaller circles branching off for the parts.
Understanding fundamental property of numbers
Singapore Math number bonds illustrate a fundamental property of numbers. Any number can be broken, or decomposed into constituent parts. While this may seem obvious to us, it is not obvious to young children. They need time to work with Singapore Math manipulatives to understand the concept of decomposition, and, after developing fluency with this, begin to write the numerals in the form of number bonds.
A number bond is a step up in abstraction from the concrete stage in the CPA learning progression and a step toward developing mental math skills. At the same time, a number bond is not as abstract as a number sentence. When we consider the CPA approach to teaching decomposition, then, we start with concrete objects. Students also benefit from placing counters into the circles of a number bond template.