十年： Shínián — ten year
树：shù — (verb) to grow, cultivate; (noun) tree
木： mù — tree; wood
百年：bǎi nián — one hundred years
人： rén — people
十 and 百 are used to mean a length of time, meaning it takes a long time to grow a tree, but even more time to cultivate a person. It is not meant to refer to a definite amount of time.
树 is usually used as a noun, meaning “tree“， but in this idiom, it is used as a verb, meaning to grow or to cultivate.