Paired t-test is used when the two scores being compared are paired or somewhat matched (i.e., they are not independent of one another) or if the two scores are repeated measures (e.g., within-groups design). An example of repeated measures is comparing the mean difference in a group of people who have taken a pretest and a posttest before and after an intervention.
Leech, N., Barrett, K., & Morgan, G. A. (2013). SPSS for intermediate statistics: Use and interpretation. Routledge.