The term 'shortage occupation' refers to sectors facing a shortage of skilled professionals. Job roles cannot be filled by residents of the UK, and hence, there is a need to fill positions with foreign skilled workers.
The ISL, or Immigration Salary List, specifies the sectors that lack skilled workers. Employers can easily sponsor foreign workers for visas for job roles mentioned in the ISL. The Shortage Occupation List (SOL0) was replaced by the ISL.