Non-standard units (paperclips, blocks, handspans) teach measurement concepts before introducing rulers. Students learn that consistent units are essential and that choosing different units changes the numerical result.
You already know how to count to 20. Measuring length is really just counting—but instead of counting objects in a pile, you are counting how many of something fit from one end to another. If you lay paperclips end-to-end across a book and count them, you are measuring the book's length in paperclips. The number you get tells you something real about how long the book is.
A non-standard unit is anything you use consistently as your measuring tool: a paperclip, a small cube, a crayon, the width of your thumb. The key word is consistently—you have to use the same-sized unit every time and line them up without gaps or overlaps. If you measure a desk with big blocks and get 6, and your friend measures the same desk with small blocks and gets 12, neither of you made a mistake. You just used different units. The desk is exactly the same length both times.
This is the big idea: a number alone doesn't tell you the length. "The book is 8 long" doesn't mean anything until you know 8 what. "8 paperclips" means something specific. "8 crayons" is a completely different length. This is why the unit is always part of the measurement. Later, when you use a ruler, the ruler is just a non-standard unit that everyone has agreed to use—so that when you say "12 inches," everyone knows exactly what you mean.
Measuring also teaches you to look carefully at a line from beginning to end. You start at one end, place your units carefully without leaving spaces between them, and count as you go. If a unit hangs over the end, the object is shorter than that unit; if it doesn't quite reach, you may need to estimate. This careful, step-by-step process—placing, checking, counting—is the foundation of all measurement, no matter what tools you eventually use.