It’s often hard to explain to people what textbooks and teachers mean by “The Tree of Life”. As life evolved from single-cell organisms, life became more and more complex. The “branches” of evolution took hold, and different lifeforms began to come about. Excluding the roots, a tree is a great representation of this evolutionary process. Just like on Earth, a tree starts from a seed and grows into many different, complex branches. Each branch can be examples of different paths of the evolutionary process. Each life represents life.
You can find a great representation of the Tree of Life here.