What is the meaning of a forbidden act not being permissible except out of necessity when there is no other substitute, and does this mean permitting masturbation out of severe necessity?

Masturbation is forbidden, and abandoning it is obligatory. However, if the matter revolves between it and something that is more gravely forbidden, then in such a case, it falls under the rubric of committing the lesser of two evils to avoid the greater, which is a principle established in Sharia. So, if a person cannot repel fornication except by masturbation, he should prioritize it over fornication, committing the lesser of the two evils.