Length or distance is a measurement of how far two points are away from each other. Typically we use length to describe the amount of space between one end of an object and an opposite end of that object. Distance is typically used to measure the space between two points on an object.
In order to measure the length of an object we need some type of measuring device. Really anything can be used to measure the length of something. For example we can measure the length of a room using out bodies. Let's say we are in a room and in our room we have a friend named Connor. We want to find the length or distance from one side of the room to the other. Connor can lie down with his feet against one wall. Now we can put a block at his head. Connor can then get up and put his feet against that block and lie down again. We can then put another block at his head. We can repeat this over and over until we get to the opposite wall.
In the picture above Connor had to lay down 4 times before he reached the opposite wall. We can say that the length of this room is 4 Connors long.
While this is fun to do, it's not really super helpful because not everyone knows Connor. If we were to call up someone on the other side of the country who we didn't know and tell them the room is 4 Connors long, they wouldn't know what we are talking about. They have never met Connor so they have no idea how long 1 Connor is let alone 4!
Typically we use standard lengths to measure. Units of length called feet, meters, inches, kilometers and miles are all well known by most of the world. Since these units are so well known people have made rulers which indicate these different lengths. We can use these rulers to find out how long something is. Let's look at a pencil for example.
If we want to find the length of this pencil in inches, we can use a ruler. Many ruler's start with the number 0 and have a number marked off every inch. To find the length of the pencil we line up the end of the ruler to one end of the pencil and then read the number that the other end of the pencil is lined up with.
In this case we can see that our pencil is 7 inches long. It is important to line up one end with the end of the ruler (or the 0 inches spot) so we get the correct measurement. It is also important that when measuring something you are looking directly at the object you are measuring and the ruler. If you are looking from the side, your eyes can play tricks on you and give you a measurement that is too large or too small.