17. Video: Does the Line Fit the Data Well?
How Do We Know If Our Model Fits Well
The Rsquared value is the square of the correlation coefficient.
A common definition for the Rsquared variable is that it is the amount of variability in the response variable that can be explained by the x-variable in our model. In general, the closer this value is to 1, the better our model fits the data.
Many feel that Rsquared isn't a great measure (which is possibly true), but I would argue that using cross-validation can assist us with validating any measure that helps us understand the fit of a model to our data. Here, you can find one such argument explaining why one individual doesn't care for Rsquared.