Length is a measurement of an object from one end to another.
Let's compare the length of the pencils.
This is a long pencil and this is a short pencil.
Here is another another example:
This is a long pencil. This is a longer pencil. This is the longest pencil.
Let's see an example of length measurement by comparing the height of two boys.
This boy is tall. But this boy is short.
This boy is tall. This boy is taller. And this boy is the tallest.
Now let's compare the width of the two jars.
This is a narrow jar and this is a wide jar.
This jar here is narrow. This jar is narrower. And this jar is the narrowest.