Absolute referencing is a way to fix a cell reference so that it doesn't change when the formula is copied to another cell. It is indicated by dollar signs ($) before the column letter and row number e.g ($A$1).
According to the data in the question being solved, what is test error?
In a scientific context, TEST ERROR typically refers to the error rate abserved when a predictive model or hypothesis is tested on a new, unseen dataset.