Prediction of RMSE

How do I know that the value obtained from the RMSE(either from linear regression or Decision Tree or Random Forest etc.) is correct ( or the best)?
I know that a lower RMSE is better. I am also aware that a value too close to 0 might be a result of overfitting.

From the housing project in chapter 2, I do not follow why a RMSE of $ 68628 was underfitting, and the value of $ 52634 is [better.](url

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