Today’s quote is a pun taken on the website Foozine. In order to understand it, you have to know the the French are very heavily taxed, that France, in fact, has a “higher tax burden than any other country in the Euro zone” .
impôt (masc.): income tax
tandis que: whereas
l’est: is (it)
English grammar would skip the “it” (l’est) altogether. the sentence would be translated as such:
… tandis qu’imposable l’est totalement.
… whereas the word ‘taxable’ totally is.