A Medium creature normally can't wield a Large 2-handed weapon
The titan fighter extraordinary ability giant weapon wielder allows, for example, a Medium warrior to wield a Large two-handed weapon in two hands. Weapons on Weapon Size says that this is usually impossible:
In general, a light weapon is an object two size categories smaller than the wielder, a one-handed weapon is an object one size category smaller than the wielder, and a two-handed weapon is an object of the same size category as the wielder.
The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder's size and the size of the creature for which the weapon was designed. For example, a Small creature would wield a Medium one-handed weapon as a two-handed weapon. If a weapon's designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can't wield the weapon at all.
Inappropriately Sized Weapons: A creature can't make optimum use of a weapon that isn't properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. If the creature isn't proficient with the weapon, a –4 nonproficiency penalty also applies.
Emphasis mine. That means a typical Medium creature that attempts to wield a Large two-handed weapon must wield that weapon as something other than a two-handed weapon, and, therefore, the Medium creature just can't wield it. However, the extraordinary ability giant weapon wielder allows a Medium warrior to employ a two-handed Large weapon but the warrior suffers an additional –2 penalty on attack rolls with it, for a total of a –4 penalty on attack rolls (the other –2 penalty coming from the weapon being inappropriately sized in the first place).
To be clear, a typical Medium creature can wield oversized weapons as follows: a Large light weapon as a one-handed weapon, a Large one-handed weapon as a two-handed weapon, and a Huge light weapon as a two-handed weapon. The warrior suffers a –2 penalty on attack rolls with the Large weapons and a –4 penalty on attack rolls with the Huge weapon.
However, a typical titan fighter adds to this: a Large two-handed weapon as a two-handed weapon. The warrior suffers a –4 penalty on attack rolls with this Large weapon.
Further, the titan fighter special ability incredible heft reduces the penalties for (but doesn't grant bonuses to) a warrior employing a weapon one size category bigger than the warrior. In other words, it's only at level 19 when a Medium titan fighter finally can use a Large two-handed weapon without penalty.