Learn French: Les chats sont rapides.

It is pronounced somewhat like "lay" in English. The "s" at the end is completely silent here because the next word (chats) starts with a consonant.

Is les used for masculine or feminine words?

It is used for both! While singular words use le (masculine) or la (feminine), the plural form for all words is simply les. You don't need to worry about gender when saying "the" for a plural noun.

Why is there an "s" at the end of rapides?

In French, describing words (adjectives) have to match the noun they are describing. Because chats is plural (more than one cat), we must add an "s" to the end of rapide to make it plural too.

Do I pronounce the "s" at the end of chats and rapides?

No, the plural "s" at the end of nouns and adjectives is almost always silent in French. When listening to this sentence, the only clues you hear that tell you it is plural are the words les and sont!