Grinding ceramic tiles into mirrors is an idiom, which means treating ceramic tiles as mirrors, which is a metaphor for the impossible. This idiom comes from the legend of Jingdezhen Lantern. It is about a Zen master who asked his disciples to take a mirror to hold water. As a result, the disciples came back and found that the mirror was broken, because the mirror could not reflect the image of water. Therefore, a mason can't be a mirror, that is to say, if he only pursues the superficial form and ignores the inner practice, it is impossible to really achieve something.
Meditation to become a Buddha is also an idiom, which means to realize the true meaning of Buddhism in the process of meditation, thus becoming a true Buddhist believer. This idiom comes from "Five Lights Meeting Yuan", which is about a Zen master who realized Tao in meditation and became a true Buddhist believer from then on. However, meditation is not a simple act, and it requires a certain spiritual foundation and understanding to gain something. If we just imitate others' actions without paying attention to inner practice, then meditation can't help people really understand the essence of Buddhism.
To sum up, the saying that grinding tiles can't become mirrors and meditation can't become buddhas emphasizes the importance of inner practice. Only by continuous efforts and serious practice can we really achieve something.