The Middle Way
The Middle Way is the path between extremes.
We can see how it emerges from the Buddha's own experience: having grown up in luxury and found that unsatisfying, he tried asceticism: the life of a wandering holy man, fasting and subjecting his body to various physical experiences intended to subdue craving and bring about wisdom. Finding this did not lead to an understanding of the truth, he came to advocate a simple and dignified life, providing just what was necessary to support a life devoted to spiritual practice.
At a more sophisticated level, the Middle Way represents the path between extreme views. Reality is ultimately indefinable, nothing can be pinned down to This or That. Most importantly, it applies to the question of nihilism and eternalism.
Buddhism says everything in life is dependent on other things, which keep changing. (See Conditionality/Dependent origination and the Three Marks.) This means that nothing is completely independent. Nothing has any fixed self. Nothing is permanent.
It would be tempting to say that nothing really exists at all. This is nihilism.
The opposite of this would be to say that everything, though mostly changeable, has something about it which lasts for ever - a soul or essence. This is eternalism.
What the Buddha saw at his Enlightenment was that Reality was neither of these. The Way Things Really Are is somewhere in between. You can't say that what we call 'a tree' exists or does not exist. It isn't really a thing. It's a process; it appears more or less the same for periods of time, but it is constantly changing.