How do you spell it in words, for example, when you must read similar formula aloud? 用 英語 (美國) 要怎麼說？

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@MoeAyman: Thanks a lot! f(xi) is a function f of the argument xi. And ^ above f means that it's our estimate of f, because we don't know a true f, we can just suppose that f is approximately equal to estimate of f.
Example at the picture: black curve - true f (usually we don't know it), dots - observations (usually it's all we have to make estimate of f), green and blue curves and an orange line are different estimates of f. The best one is blue curve, because it's closest one to true f, but green curve have the smallest MSE, so it's closest to observations (dots). There is a problem between blue and green curves, becase computer can estimate f like green curve, according to the smallest MSE. But it's not true form of function f. This problem is known in statistical learning (or machine learning) as overfitting the data.

@MoeAyman: Yeah, I really appreciate your help! Thank you!

@Switch Thank you! I hurried to the choice of a better answer. Your answer is more appropriate.