Metabolic stress is simply the build-up of metabolites (mainly lactate) in the muscle during fatiguing exercise. This can increase muscle activation and promote growth signals that can improve overall gains! BFR training and light load/high rep sets are great ways to promote metabolic stress. Check out our article, “What Causes Growth?” here for more info.
Reference: Wackerhage, H., Schoenfeld, B. J., Hamilton, D. L., Lehti, M., & Hulmi, J. J. (2018). Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise. Journal of Applied Physiology.