Literally: one – particle – ten thousand – times (as in multiplication)
Alternately: From something tiny, a huge profit. By extension, an admonition not to look down on small or simple things, because even the most humble appearance can hide vast potential. Also, it seems, a metaphorical appellation for the rice plant, which grows from one grain and produces countless more. And finally, this phrase is a Buddhist teaching that one good deed, no matter how small, can produce myriad good results in the world.
Notes: This saying apparently comes to us from a Buddhist sutra known in Japanese as the 『報恩経』(Houon-kyou), short for 『大方便仏報恩経』 (Daihou benbutsu houon-kyou), literally “Great teaching Buddha gratitude sutra” – if it has a more poetic or standard English name, I haven’t found it yet.