The story of Philoctetes is in Sophocles' play Philoctetes (not
in the Iliad).
Philoctetes had inherited Heracles' magical bow which according
to a prophecy the Greeks needed in order to be able to conquer
Troy. (Philoctetes' bow eventually killed Paris - one of the heroes
who needed to be killed before the city could fall).
Since Philoctetes had been marooned on the island of Lemnos
during the voyage to Troy (Philoctetes' had a septic wound, the
Greeks could no longer stand the sight or the smell of it) Odysseus
and Diomedes set out for Lemnos to persuade Philoctetes to lend his
magical bow.