Characters per line refer to the number of letters, numbers, and symbols that can fit in a row of words on a page or a screen. Imagine the words you see on a book or an article on the internet. When you read them, you can see that the words are not just placed anywhere, but they are arranged in a way that they fit in a line with other words.
Characters per line is kind of like fitting people in a row for a picture. If too many people are in the row, then they might feel squished and uncomfortable, and you won't be able to see them clearly. It's the same with characters per line – if there are too many letters in a row, the words might feel squished and uncomfortable to read, making it difficult for you to understand what's written on the page or screen.
That's why writers and designers decide how many characters should be in a line. They consider things like the font size, page size, and the type of content they are writing. For example, a book for children might have a smaller number of characters per line than a textbook for adults.
To see an example, look at the text on your phone or computer screen. Notice how there is a certain amount of space between the left and right edges of the screen, and how the words fit into that space in neat and tidy rows. That's the number of characters per line that the designers chose to make it easy for us to read what's on the screen.