Partitioning numbers is taught in Years 2/3 and involves separating numbers out into units, tens, hundreds, etc. Once children have mastered partitioning 2 digits (tens and units), they’ll move onto 3 digit partitioning (hundreds, tens and units).
It’s important for calculation because it helps children break down larger numbers into numbers that are easier to work with. I remember learning partitioning, sat at a desk with endless worksheets. However, with a few items you can make a fun and interactive way for children to learn 3 digit partitioning.