Using a ruler with standard units (inches or centimeters) introduces precise, conventional measurement. Students learn to align the zero mark and read the correct ending point accurately.
Before this, you measured length with non-standard units — maybe paper clips, or cubes, or your hand. You would line them up end to end and count how many fit. That worked! But there was a problem: if you used your paper clips and your friend used their paper clips, and they are a different size, you would get different answers for the same object. That is why people invented standard units — units that are always the same size, everywhere, for everyone.
A ruler is a tool with those standard units already marked on it. The marks are called inches or centimeters, depending on which side you use. Every inch is the exact same length as every other inch. So if you say something is 5 inches long and your friend measures it and also gets 5 inches, you know you got the same answer because you used the same-sized units.
The most important rule when using a ruler is: line up the zero mark with one end of the object. The ruler starts counting at zero, not at the edge of the plastic or wood. If you start at the wrong place, your whole measurement will be off. Put the 0 right at the very start of what you are measuring. Then look at where the other end of the object lines up with the numbers. That number is the length.
Reading the ruler means looking carefully at which number the end of the object touches. If it ends right at the 4, the length is 4 inches (or 4 centimeters). If it ends between two numbers, you might say "a little more than 3" or "almost 4" — for now, it is okay to read to the nearest whole number. The key habits are: zero at one end, read the number at the other end, and check that the ruler is lying straight along the object, not crooked.