How do you tell if a confidence bound for a constant (Pi) for a polynomial fit "overfits" the data?

In Matlab's curve fitting toolbox help section, specifically the interactive curve fitting example, subheadings:__interactive curve fitting procedure, determining the best fit,_there is a picture of the fit editor that has a fifth degree polynomial equation with constants P1, P2,,, etc. The second paragraph text states that P1, P2, & P3 "overfit" the census data.(The data was obtained via the command," load census")I don't understand how you can determine whether a confidence bound overfits the data. In addition, how do you determine whether a confidence bound does not overfit the data? If there is a way to download images please let me know how to do this.

Thanks