Learn French: Les filles sont toujours timides.

Because it describes les filles (the girls), which is a plural noun. In French, adjectives must match the number of the noun they describe, so we add an 's' to the singular word timide to make it plural.

Since filles is feminine, do we need to add an extra 'e' to timides?

No, we don't. The base singular form of this adjective already ends in an 'e' (timide). When a French adjective naturally ends in an 'e', it is used for both masculine and feminine nouns. To make it plural, you simply add the 's'.

Why does the word toujours end in an 's'? Is it plural to match the girls?

No, toujours (always) is an adverb, which means it is a fixed word whose spelling never changes. It just naturally has a silent 's' at the end of it as part of its base spelling.

Do I pronounce the 's' at the end of filles and timides?

No, the plural 's' at the end of most French words is completely silent. When listening, you mainly know the sentence is plural because you hear les instead of la, and the verb sont instead of est.