Overfitting: when your model is like a clingy ex, it just won't let go. It's like they say: "I'll train you, but only if you do exactly what I want, or else I'll just throw a tantrum!"
Avoid overfitting like you'd avoid a math homework assignment: use a lot of regularization, don't be too attached, and for the love of all that is sane, don't look back at your code for 5 years.
Want to learn more about overfitting? Visit our Regularization Rodeo or our Early Stopping Showdown subpages for some expert tips.