Unitizing is the counting and quantity principle that refers to the understanding that you can count a large group of items by counting smaller, equal groups of items from within the large group.
This is a pretty heavy concept. When we first learn how to count, we are counting individual objects which are all relative to one and thus we sort of ignore it to avoid redundancy and unneeded complexity at that time. But when we start to make equal groups and count those groups for place value, skip counting, and eventually multiplication, things get complex fast and it all starts with the ability to unitize.
Luckily, I have a whole playlist of Counting and Quantity Principle posts you can access right now.
For a full summary of unitizing and all the other counting principles, read the original blog post from my personal blog, Tap Into Teen Minds.