This is described in this wiki article that was linked in another comment.
https://en.wikipedia.org/wiki/Learning_curve
Which also points out how people rarely use the term properly, and call something difficult to learn a "steep" learning curve. It's the opposite.
“The common English usage aligns with a metaphorical interpretation of the learning curve as a hill to climb.”
Followed by a graph plotting x “experience” against y “learning.”