Imagine you have a magic box where you can put any story or book inside it. This box will count and separate each word from the story and make a list of the most frequently used words in that story.
For example, if you put in the story of "The Three Little Pigs", the magic box will count how many times the word "pig" appears in the story and put it at the top of the list. The same goes for other words like "house" and "wolf".
The word frequency is simply the number of times a word appears in a story or written text. The more often a word appears, the higher its frequency. If a word has a high frequency, it means that it is an important part of the story and is used a lot by the author.