Here is a visual example for overfitting: you can see that the overfitting model on the right is just too complex and it is oscillating too much in certain areas. These are the areas the model does not generalise sufficiently and might cause big issues:
Source: https://kluedo.ub.rptu.de/frontdoor/deliver/index/docId/6867/file/_Karatas-Comparison_of_model_based_methods_with_machine_learning_strategies-3.pdf
Especially high order polynomial fits can suffer from this oscillating / overshooting, especially in the edge regions, see also:
Source
Please let me know if this answers your question, @tennis_geek.
Have a good one! 
Best regards
Christian