What does this actually mean?
Rain does not discriminate where it falls, it sheds its wetness on all things equally. The outer weather nourishes our inner life, makes us grow into compassionate loving individuals as we age and broaden with experience and wisdom. If we try to protect ourselves from it with artificial tools, like man-made umbrellas, we do not grow, we remain stunted in our adolescence for all eternity. Remaining in the material world of social rules and morays we never move beyond competition, greed and self-centredness.
Money is one readily available umbrella, where the rich think they are sheltered from rain, rain, that is a metaphor for Life. And to some extent they are outwardly, materially, because they can more easily maneuver society and be more comfortable as they are doing it but what happens to their inner world. They start to think Life is unfair and that is acceptable, the poor can remain homeless because Life made it that way, "dog eat dog" and they were chosen by Life for a better existence...etc. You all know the script.
But Zen says no, you are wrong and stunted in your growth and thinking, Life tries to nourish all, it does not play favourites and karma exists. Life, beside matter, is illuminated by the miracle of awareness which will live long after our body and money disintegrates. And the sooner we start to appreciate the message of rain and start enjoying it's luscious fruits, rather than whine about its wetness, the sooner our souls will soar.