The Wilcoxon Signed-Rank Test is used to find out whether two related measurements of the same group can be considered statistically different or not. These two measurements typically represent different times (e.g. a pre- and post measurement with a treatment between them) or two different but related conditions. The Wilcoxon Signed-Rank Test needs two related variables that both are measured on the exact same scale at ordinal level or higher. It is also the preferred test when your data violates the assumptions of a Paired-Samples T-Test.