In the Star Wars universe, the light side of the Force, also known as Ashla, was one of two methods of using the Force. It emphasized selflessness, living in the present, clearing the mind, letting go of the consciousness, acting on instinct, listening to the Force and following its will. In contrast, the Sith, who follow the Dark Side bend the Force to their will.
To successfully follow the will of the Force, Jedi are detached from certain emotions, whether positive or negative. The light side was to be used for knowledge and defense, and channeling it required a Jedi to be calm and at peace. Negative emotions such as anger, fear and jealousy were fuel for the dark side, which offered a quicker, seemingly easier path to great power. However, according to Yoda the dark side was not more powerful than the light side in and of itself, but was merely "quicker, easier, more seductive". To avoid the dark side’s lure, the Jedi developed a code emphasizing detachment and forbidding emotional attachments. They were encouraged to love in a pure way that doesn’t involve jealousy or obsessive behavior.
Despite the above, according to this site, the light side was never mentioned in the movies while the dark side is mentioned frequently. This will not be the first time that wrongly remembered lines become canon.
Side Quest 2024.