Reach is a mod that increases the range of a melee weapon's attacks, except for the throw distance of glaives (they can hit enemies more easily, though, because the projectile itself has a greater reach). Reach also increases the range at which Helios can attack enemies if installed on Deconstructor.
This mod does not increase the radius of slam attacks.
This mod does not increase the range limit of all thrown melees, but instead grants said weapon a larger hitbox.
The effects of Reach are not seen in the energy trails that appear while swinging a weapon, but the trails themselves determine which angles would the attacks land as the new range of the equipped weapon.