In a typical stress-strain curve, stress is defined as the applied force divided by the original cross-sectional area of the test specimen, and strain is defined as the change in specimen length divided by the original length. But these are just approximations of the stress and strain in the specimen. They are called engineering stress and engineering strain.
True stress and true strain are the actual stress and strain in the specimen, that account for the fact that the dimensions of the specimen change throughout the duration of the test. The video below explores the topic of true stress and true strain in more detail.