Learn French: Le chef aide tous les employés. - The boss helps all the employees.

The word for "all" changes its ending to match the noun it describes. Because les employés is masculine and plural, you must use the masculine plural form, which is tous.

Is the "s" in tous pronounced in this sentence?

No, the "s" is silent here. When tous is placed before a noun to mean "all the" (as an adjective), the "s" is silent. You only pronounce it when tous stands alone to mean "everyone" (as a pronoun).

Doesn't the verb aider usually take the preposition à?

Only when you are helping someone to do an action (aider quelqu'un à faire quelque chose). When you are simply helping a person, it takes a direct object with no preposition. That is why it is directly aide tous les employés.

Does chef always mean "boss" in French? I thought it meant a cook!

In French, chef broadly means "boss", "chief", or "manager" in almost any workplace. An actual kitchen chef is formally called a chef de cuisine (boss of the kitchen), though they just use chef for short in a restaurant setting.