Spanish · Pronunciation Guide
FREE-oh, two beats — the accent breaks the i and o apart so they do not glide.
Two syllables, FRÍ-o, and the accent does structural work rather than simply marking loudness. A weak vowel like i next to a strong one like o would normally form a diphthong and give a one-syllable "frio"; the mark forces a hiatus, splitting them into separate beats and placing the stress on the first. The feminine "fría" works the same way.
Listen: frío
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El agua está muy fría.
The water is very cold.
Listen: El agua está muy fría.
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"El agua está muy fría" shows the gender puzzle of "agua": it takes "el" in the singular purely to avoid two a sounds colliding, but it is genuinely feminine, which the adjective "fría" reveals. The accent on "fría" splits the i from the a, exactly as it does in the masculine form.
Readers collapse the two vowels into one syllable, producing "fryoh", which is precisely what the accent forbids. Say "FREE" and "oh" as two distinct beats. The r then wants the English curl instead of a single flick. And there is a vocabulary point worth flagging: "tengo frío" means I am cold, while "estoy frío" would suggest you are cold to the touch.