Tathagata means “thus come/gone” and is a way that the Buddha was referred to. It is a way of indicating his nonduality because he is come and gone at the same time without them interfering with each other. It is like the talk in Buddhism of non-arising, non-vanishing. It is more or less the dual of that characterization of enlightenment. That is a way of talking about the Heraclitian flux of change in existence, in which change changes, i.e. variety happens always and is constantly renewed. The Buddha is gone in the sense that his self has been annihilated or canceled into emptiness. But he is come in the sense that he can articulate the Dharma, as all prior and coming Buddhas have. And since all Buddhas point to the same truth of existence, Shakyamuni is just one more instance of what as gone before and what will come after in terms of those pointing to the middle way which is intrinsically nondual.
By the way, who ever achieves enlightenment is called a Tathatagata, it is not an exclusive club with only Shakaymuni and Maitreya (who has not come yet) are part of. Enlightenment is part of our heritage as humans, it is open for anyone to achieve in a way that is unique to themselves. It is the realization of the quintessence of our humanity that was there primoridally, and which the Buddhas only remind us of when they arrive already departed.