Titans are the elder gods who used to rule before they were dethroned by the younger gods, the Olympians.
When the Titans were dethroned, Zeus banished the Titans to Tartarus. A place that is deeper than the Underworld and is dark and gloom. It is so deep it would take a anvil nine days to get there if it is dropped. Tartarus is surrounded by a bronze fence made out of iron making it the ultimate place of imprisonment.
Their plan is to get revenge on the Olympians for ever banishing them to Tartarus. They plan to find the best battle strategy to not only end the war, but to conquer Greece and win it.