If you don't know what a yew tree looks like, imagine the most Goth-looking Christmas tree ever. It has gnarled bark, a short trunk, and needles and berries instead of big flat leaves. Its branches spread wide like a weeping willow. In other words, it's a perfect tree to plant in your cemetery, and a perfect tree to turn into a monster.
But yew trees are more than just spooky in A Monster Calls. For one thing, they've got some major healing powers, as they're used to make the chemo drug Conor's mom's doctors prescribe in a last-ditch effort to save her life. Too bad the yew tree drug doesn't work. Conor's just as disappointed as we are, as evidenced by his kicking the tree after he visits his mom and yelling, "'What's the use of you if you can't heal her? Just stupid stories and getting me into trouble and everyone looking at me like I've got a disease—'" (27.27).
Conor's got a point. Every time Conor thinks the monster's on his side, it does something else to get him in trouble, like wrecking his grandma's living room and beating up Harry. So maybe all those healing powers the yew tree is supposed to possess are nothing but a load of bullpucky.
True to the constant plot twists of A Monster Calls, the yew tree is Conor's shelter the night his mother dies. The tree may not heal his mother, but it heals Conor, or at least helps start him down his own road to healing (thanks to a healthy dose of Facing the Truth). If there were a moral to this story, and if the yew tree's significance could be summed up in a song lyric, it would totally be by the Rolling Stones: "You can't always get what you want / But if you try sometimes / You just might find you get what you need."