This is damage to a tissue caused by stress/loading (too much at one time, not prepared for the stress, or repetitive use). This stress can either be to the muscle itself (the red part in anatomical figures) or the tendons that attach muscle to bone (the white part in anatomical figures).
Depending on the extent of the damage, you could have actual tearing of muscle fibers. A mild strain (grade I) would be stretching of the tissue with possible small tears to fibers. There will be pain, but there will be minimal weakness. A more severe (grade II) strain would involved tearing of fibers. Weakness would accompany the pain. The most severe form (grade IIII) is where most or all of the fibers are damaged. There could be a complete rupture of the muscle. There will be a loss of muscular function.