To understand roots, think of a seed. Our causes are seeds that we plant. They might be good, bad, or pure. When conditions (circumstances) are right, the seeds will grow. Let's say we hear the name of Amitabha Buddha. A seed is planted deep in our Alaya consciousness. We then chant the name. The seed is strengthened and grows. Eventually, the seed can grow into a sapling, then into a tree. As it grows, the tree's roots grow stronger and deeper.
With our Pure Land tree strong and its roots deep, we have "good roots."
With good roots, we continue to return to our practice over many lifetimes. We are drawn to the practice for we have a strong affinity for it. With good roots, conditions naturally occur as those conditions mature. The more we practice and learn, the stronger our "good roots."
We are both happier now and closer to being reborn in the Pure Land in the future.