Learn French: Ces crevettes sont toutes froides. - These shrimp are completely cold.

Adverbs usually don't change form. However, tout has a special rule. When it means 'completely' or 'very', it must agree in gender and number if it comes right before a feminine adjective that starts with a consonant or an aspirated 'h'. Since froides is feminine plural and starts with a consonant, tout becomes toutes.

What would happen if the adjective started with a vowel?

If the feminine adjective starts with a vowel or a mute 'h', the adverb tout stays exactly as it is (invariable). For example, if they were 'completely enormous', you would say Ces crevettes sont tout énormes, not toutes énormes.

How do I know if the 's' at the end of toutes is pronounced here?

The 's' at the end of toutes is silent here. You just pronounce the feminine 't' sound at the end (it sounds like 'toot'). You would only pronounce the 's' as a 'z' sound if the next word started with a vowel, creating a liaison, but froides starts with a consonant.

Could toutes mean 'all' instead of 'completely' in this sentence?

Yes! Out of context, this sentence could also mean 'These shrimp are all cold.' French uses the exact same structure for both meanings. You just have to rely on the context of the conversation to know if the speaker means the temperature is completely cold or that every single shrimp is cold.