When you decompose a number into its various addends, you are breaking that number apart into different sets of other numbers (addends) that can be added together to get the original value.
To decompose in a math problem, separate the digits into their place values. Then, start by adding like values. For example, 62 + 37 decomposed is 60 + 30 + 2 + 7 which equals 97. 62 + 37 = 97