What Is Underfitting?

Underfitting is the opposite problem: the model didn't study enough and never really understood the pattern.

Imagine it like this: A student who barely opened their textbook and only remembered a single very basic fact will do poorly both on practice problems and on the real exam — because they never really grasped the pattern in the first place.

Examples:

• Trying to predict a house price using only "is the front door blue?" — a clue that's way too simple
• A model with far too few examples to find a real pattern

So? Underfitting means the model is too simple or didn't get enough good examples — the fix is usually better features or more training.