From Sots2

A cabled harpoon system that is designed to slow or immobilize enemy starships, making them easier targets for weapons-fire and boarding craft. Successful use of a grappler-line can put the attacking ship in a similarly vulnerable position as the target ship, as they are attached to another ship just as much as the other ship is attached to them. Targets should be chosen wisely.

The research of Grapplers opens the possibility to research the following technologies: