Basic was the most widespread language in the galaxy and was spoken most often by humanoids. Some species were physiologically incapable of speaking it, such as Wookiees and Ithorians, though they could use translators to communicate with Basic-speaking species.
Astromech Droids were meant to work with machines, and thus were typically not outfitted with complex vocalizer units, also making them unable to speak Basic.
Species that could not speak Basic could learn to comprehend it audibly. If Basic-speaking beings around them also knew their alien or droid language, this could allow two-way conversations. For instance, Ithorian woman Oora could speak with Human Kanan Jarrus because he understood her language.
Basic appears in every Star Wars Rebels work. This is a list of appearances in which the language was directly mentioned.