When you take a reading using a ruler, this is called the measured length.

In the picture below you can see a ruler being used to measure the length of the toy van.

It is important to distinguish the measured length from the actual/true length as some measurements are better than others.

The measured length may differ from the actual/true length.

Here is another example:

There is an actual/true value for the volume of liquid in this measuring cylinder. When you take a reading from the scale, you get a measured value. This value should be very close to the true value of the volume if you have set up the measuring cylinder correctly and read the scale properly.