Learn French: Le chef comprend tous les détails. - The boss understands all the details.

In French, "all" has to match the gender and number of the noun it describes. Since détails is masculine and plural, you must use the masculine plural form, which is tous. If it were a singular masculine noun, you would use tout.

Is the "s" at the end of tous pronounced in this sentence?

No, the "s" is completely silent here. When tous is placed before a noun (like tous les détails), it acts as an adjective and the "s" is dropped in speech. You only pronounce the "s" when tous stands alone as a pronoun (meaning "everyone" or "all of them").

Wait, doesn't le chef mean "the cook"?

It can, but in everyday French, le chef is the standard word for "the boss," "the manager," or "the chief." The English word "chef" actually comes from the French title chef de cuisine (head of the kitchen).

Why does comprend end in a "d"?

This is the third-person singular form of the verb comprendre (to understand). It is an irregular verb that follows the exact same pattern as prendre (to take). In the present tense, the singular forms keep the "d" from the infinitive, and it remains silent.