An offer letting existing shareholders buy new shares at a set price, usually below market, in proportion to what they hold. You have three choices and doing nothing is the worst of them: subscribe and keep your ownership percentage, sell the right if it is tradable and collect its value, or let it lapse and be diluted for nothing. The discount is not a gift, since the new shares dilute the old ones.
See every term in the full PSE investing glossary.