Yes, the post title is an obscure Passover joke; thanks for asking!

足るを知る者は富む
(Taru wo shiru mono wa tomu; “Who knows ‘enough’ is wealthy.”)

Definition:

Even a shoestring budget is wealth if it’s enough. People whose needs are met and who are satisfied with what they have are truly rich, even if their finances seem meager. Happiness, life satisfaction, and spiritual fulfillment do not depend on money.

Breakdown:

Let’s work left to right today. We start with the verb 足る (taru), “to suffice.” It could be in sentence-final or prenominal form, but I suspect the latter because it’s followed by the direct-object-marker particle を (wo) as if it were a noun, and we’ve seen cases before in which this form allows verbs to behave like nouns. The verb that takes taru as its object is 知る (shiru), “to know,” “to understand”; this verb too is in prenominal form, because everything so far is a relative clause modifying the noun 者 (mono), “person.” That person is marked as our topic of discussion by the particle は (wa), and the entire comment given on that topic is the verb 富む (tomu), “to be abundant,” “to be rich,” in sentence-final form.

The phrase 足るを知る by itself can be used to mean “to know that one has enough” or even “ to be happy with one’s lot in life,” giving us a fair amount of leeway in translating this phrase.

Notes:

This is yet another saying that descends from our old friend Laozi, part of a longer passage that also asserts that “Who works hard has willpower.”

If you try to type this into a computer, be careful that taru isn’t rendered as 樽, “cask” or “barrel.”

Example sentence:

「確かにそうですが、大金持ちが『足るを知る者は富む』と繰り返して言い張るのは、何とも言えません」

(“Tashika ni sou desu ga, ookanemochi ga ‘Taru wo shiru mono wa tomu‘ to kurikaeshite iiharu no wa, nan to mo iemasen.”)

[“It’s certainly true… and yet, for someone super rich to keep on declaring that ‘true wealth is satisfaction with what one has‘ leaves me speechless.”]