Residual Sum of Squares is a measure of the discrepancy between the data and an estimation model. A small RSS indicates a tight fit of the model to the data.
Residual Sum of Squares is used as an optimality criterion in parameter selection and model selection.
In general, total sum of squares = explained sum of squares + residual sum of squares. For a proof of this in the multivariate Ordinary Least Squares (OLS) case, see partitioning in the general OLS model.