Essentially, think of using your laptop, but instead of looking at a monitor you're reading the output from a printer. So if you type `ls`, you see the directory listing being literally printed out on a dot-matrix printer. Naturally the only way of text editing is with a line EDitor, which lets you edit text one command at a time.

When trying to use ed on a video terminal, it obviously looks wasteful. But that's because it was designed before video terminals were a thing. Kind of like using a modern day graphics card to play pong.

here is a video illustrating using a teletype:

https://www.youtube.com/watch?v=2XLZ4Z8LpEE