Predicted value (y-hat)

The predicted value is the height of the regression line above a chosen $x$, not a value anyone observed. You get it by substituting: with $\hat{y} = 12 + 3x$ (read y-hat), an $x$ of 4 gives $\hat{y} = 12 + 3(4) = 24$. If the individual with $x = 4$ actually had $y = 26$, the residual is $26 - 24 = 2$, so the model underpredicted by 2. Predicted values stay trustworthy only inside the range of $x$ used to fit the line; outside that range you are extrapolating.