Mean Squared Error (MSE) - A cost function

Mean Squared Error (MSE) is a numeric value that represents the error of the prediction of the model.

Draw a line and calculate the distance(error) between the line and the data points.
The distance from the line to the data point is also called a "residual".

This errors are the delta between the predicted line and the data points. 

Summarize the square's of all deltas and divided the result by the number of points.

And so we can write