Words in a language are like toys in a box. Just like some toys may be bigger or heavier than others, some words can be longer or more complicated than others. Word metric is like a special ruler that we use to measure these words to compare them to each other.
Now, imagine that you have two boxes of toys - one with big and heavy toys and one with small and light toys. If you try to carry both boxes, it may be difficult because one box is heavier than the other. Similarly, when we write something, it may be harder to read if we use a lot of big and complicated words.
Word metric helps us understand how difficult it may be to read something by giving us a score that tells us how complicated the words in a piece of writing are. This helps us choose words that are easy to read and understand for others.