This proverb means that we often get treated the same way we treat others. It is usually negatively, as a warning, or when a person who acted immorally gets their comeuppance. It could also be used as a promise of blessings to those who do good.  A third possible meaning is that Often compared to the Hindu doctrine of karma, the exact origin of the proverb is uncertain, but it seems to have emerged in the US in middle of the last century.

Related proverbs and quotes:
Shakespeare 

Bible:

Chinese:

German

Today's proverb is often used in Swahili to describe a person who makes a mistake that could have been foreseen and suffers negative consequences... like the truck driver in this picture from Oxfordshire, UK. Regardless of what your parents taught you (or failed to teach you), you will eventually have to confront the harsh realities of life and learn from experience.
See also: If a child cries for a razor, give it to him (Mtoto akilia wembe, mpe)

He who is not taught by his parents is taught by the world. (Asofunzwa na wazazi, hufunzwa na ulimwengu)
Here's a poem by the poet Akilimali Snow-White about this proverb. (My translation from the original Swahili)

In the age they fooled me, my old folks in raising me,
I failed to learn the new movements of the world.
Today I please myself, to the people of the world, listen:
He who is not taught by his parents, is taught by the world.

I couldn’t have done any work without humbling myself before them,
Obeying to flatter them, then to serve them,
Even when I pleased them, they taught me with intention,
He who is not taught by his parents, is taught by the world.

Now I can speak European languages without difficulty,
like English and others too,
With effort I learn, and even they have raised me.
He who is not taught by his parents, is taught by the world.

I can converse without blemish,
And lead amidst evil, removing the blemish,
In the end the place pleases, one step towards harmony,
He who is not taught by his parents, is taught by the world.

There is nowhere I have overlooked, without investigation,
All sides examined, knowledge I have taken,
I even know how to sell products and buy,
He who is not taught by his parents, is taught by the world.

The amount which I have learned, not a little by fumbling,
I am pleasing where I come from, I employ good work
It’s hard to scorn, how it raises me,
He who is not taught by his parents, is taught by the world.

It’s not right to ignore what you don’t know
Try to investigate, and then analyze,
When your intention is tightened, you can’t fail to know a thing,
He who is not taught by his parents, is taught by the world.

The tasks I taught myself, my father didn’t know
He didn’t know English, or selling and buying,
but only praising oneself, that was when I, the child, knew,
He who is not taught by his parents, is taught by the world. 

I give more for you, you all who helped me,
All of you who’ve taught me, Lord give you health
God fill you all with happiness, and return goodness to you,
He who is not taught by his parents, is taught by the people of the world.
- Diwani ya Akilimali

What do you think about this poem? What does it mean? Can you improve the translation?

Fikeni E. M. K. Senkoro (1988) wrote of this poem (my translation):

Kadiri hamu ya kula inavyoongezeka, chakula kina ladha bora.

Methali hiyo ilionekana kwa mara ya kwanza katika kitabu cha Miguel de Cervantes Don Quixote, kilichochapishwa mwaka wa 1615 (katika Sehemu ya II, Sura ya V)

Mara nyingi wazazi husema hivi kwa watoto wao wanapokuwa walaji kwa fujo.

Umewahi kuona mhunzi akifanya kazi? Au labda ulimwoma fundi akitengeza glass (kioo)? Si ni ajabu sana? (Ukitaka kuona kwa macho yako, tembelea Shanga Foundation Arusha, au tazama videos kwenye links hapo chini - ona Rasilimali).

Katika uzoefu wetu, glasi ni ngumu, yaani haikunji kabisa. Ukitumia nguvu zako zote, kio kitavunjika mkononi mwako na kukuumiza. Lakini hakika kioo hutengenezwa kwa kuyeyusha mchanga, mabichi na laini kama udongo.

Maishani kuna mambo ambayo yanaonekana kuwa magumu, yaani hayabadiliki kabisa, hayapindi. Tukitumia nguvu zetu zote, yataharibika tu na kutuumiza. Lakini fundi mwenye ujuzi anaweza kuyafanya kuwa mepesi na laini, kwa kuyatayarisha ipasavyo, na kuchukua hatua sahihi kwa wakati ufaao.

Methali hii hutumika sana kwa maana "chukua hatua haraka fursa inapotokea, ili usiikose." Kama WaSwahili wanavyosema "Samaki mkunje angali mbichi." Ona pia There is a tide:

Hata hivyo, ikumbukwe kwenye tamthilia hii, ushauri huu ulikuwa na madhara mabaya kwake, maana Buruto hakushinda baada ya hotuba hii (soma zaidi...)

Lugha na tamaduni nyingi zina methali zinazofanana sana na hii. Labda methali hizo zina chimbuko nyingi tofauti zisizotegemeana.