Force nexus is a term used to refer to any location where the Force, or any particular aspect of the Force, was unusually strong.
Such nexuses could be caused by any number of different events. Korriban in general and the Valley of the Dark Lords specifically were nexuses of the dark side of the Force due to the long-term presence of the Sith Lords, Nam Chorios was a nexus of the Force due to the omnipresence of the Tsils, a form of life, Vjun was a nexus of the dark side due to the horrible events of its past, the Dark Side Cave on Dagobah and a spot in orbit over Endor were tainted by the death of dark side users, Byss was a nexus of the dark side of the Force because it housed Palpatine's clones and his various Sith Alchemy experiments and the Force energies sucking the lives on the planet.
Their may have been a nexus in Lumiya's habitat asteroid where Alema Rar could draw energy from it to power her force specters.
The natural mountain on which the foundation of the Jedi Temple on Coruscant rested was a nexus for unknown reasons, but is was likely because of all the Jedi that had lived there for thousands of years. This became tainted by the dark side with the death of Lord Nyax at the site of the ruined Temple during the Yuuzhan Vong War. The most powerful nexus was the Valley of the Jedi, where the spirits of several thousand Jedi, both light and dark, were trapped after the Seventh Battle of Ruusan, a perhaps limitless source of Force energy even after the spirits were freed.
Objects left in the presence of a force nexus for extended amounts of time, eventually became saturated with the force. Imbuing the object, say a weapon, with special abilities, like an edge that will never dull, the ability to absorb some force powers like force lightning and electric judgement, or even the ability to enhance the wielders perception of time, allowing him to think and react faster than possible. This effect allowed even a being who wasn't force sensitive to use the weapon to a great degree. Also, objects saturated with either the light or dark side of the force, when used against the opposite force it had been imbued with, say it had been imbued with darkness. Then it would have a greater damaging effect than usual against a lightsider. And would physically and mentally weaken the lightsiders bond with the force just by being in the vicinity of the weapon or object. The same went for more mundane objects like stones or even plants and animals. In the presence of a Force Nexus, they were known to either corrupt or further enhance an objects or beings innate abilities.
