R-squared is a number between 0 and 1 that indicates how well a regression model can explain the relationship between the two variables in the model, or how well the data fits the model.

An integer multiplied by itself gives a square number. 5 squared, 5² = 5 × 5, is 25. 25 is a square number. 8 squared, 8² = 8 × 8, is 64. 64 is a square number.