Today's proverb is: A rolling stone gathers no moss. This is a well established English proverb, being included, for example, in the 16th-century proverb collection of John Heywood: The rollyng stone neuer gatherth mosse.
Metaphorical proverb. "A rolling stone gathers no moss" is a typical metaphorical proverb. That is, the proverb is literally true when it comes to stones that roll - they do not gather moss because they are in motion (this literal truth was actually tested in an episode of MythBusters). The value of the proverb comes in its metaphorical application, however, when you figure out just how that rolling stone is an emblem for the human condition. The idea is that if you stay in one place, you will gather moss; if you move, you won't gather moss.
But is gathering moss a good thing or a bad thing? In its metaphorical application, this proverb can go in two opposite ways! That makes it a really fascinating proverb, and the Wikipedia article about this saying addresses both lines of interpretation.
If you think of moss as being stuff that can accumulate during your life - stuff like a home, a family, money, possessions, etc. - then the proverb warns you about the dangers of rolling around. Someone who is a rolling stone cannot accumulate those things. If someone keeps moving around, they won't have a home or a family or any of life's trappings that are widely regarded as signs of success. You can see that negative sense in this use of the proverbial metaphor: As the rolling stone gathers no moss, so the roving heart gathers no affections. Remember that song "Papa Was a Rollin' Stone" by The Temptations? You can read about the song in this Wikipedia article; here's the chorus:
Papa was a rolling stone;
Wherever he laid his hat was his home;
When he died, all he left us was alone.
In a more recent song evocation, compare this elegant allusion in the Dave Matthews Band title "Busted Stuff," which features these lyrics: A rolling stone gathers no moss, but leaves a trail of busted stuff.
But what if the moss is a bad thing? What if moss stands for something like rot or stagnation? That is how I usually understand the proverb (which probably says a lot about me, ha ha). If you cannot keep moving, if you cannot stay fresh, if you settle down, them you will get covered with moss. Ugh: who wants to be covered with moss? Not me! According to this interpretation, the moss is what accumulates when things are forgotten, neglected, discarded, etc. You can find this sentiment already in Langland's Piers Plowman: Selden Moseth the Marbelston that men ofte treden (Seldom grows mossy the marble that men often walk upon).
Parallels. In Erasmus' Adagia 3.4.74, you can find a similar Greek saying, Λίθος κυλινδόμενος τὸ φῦκος οὐ ποιεῖ, which Erasmus renders in Latin as Saxum volutum non obducitur musco, "a rolled rock does get covered with moss." Erasmus endorses the negative interpretation, equating the saying to a proverb about plants and their roots: "A plant often moved does not take root."
Cobham's Dictionary of Phrase and Fable cites a French paralllel, Pierre qui roule n’amasse jamais mousse, and also an Italian parallel, Pietra mossa non fa muschio.
Finally, here is a poster I made for this image which definitely leans towards the negative interpretation: those poor shoes have been badly neglected and are not something anyone will want to wear again! Details about the poster here: