English is full of compound adjectives, which may combine an adjective with either another adjective, a participle, or a noun. When numbers or fractions are used to describe a noun, this can also result in a hyphenated adjective.

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Generally, compound adjectives should be hyphenated when they come before a noun (attributive position):

But beyond the well-known attractions.

Caption 20, Greentraveller: The Mendip Hills, Somerset

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This is the high-pitched noise that's been on since the beginning of the talk turning off.

Caption 50, What is theater capable of?: Simon Stone at TEDxSydney

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She was surprised that a good-looking prince was standing there.

Caption 27, Fairy Tales: The Frog King

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For weeks now, this slow-moving crisis has made it difficult—if not impossible—to move barges down the river.

Captions 28-29, Slow News with Sigrid: Climate Change and the Mississippi River

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When the adjective follows the noun (predicative position), it won't be hyphenated: 

But perhaps less well known is the crucial role they play in cooling and cleaning the world's atmosphere.

Captions 14-15, The Prince's Rainforests: Project Prince of Wales' Introduction

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Adjectives coming before the noun that involve numbers and units will generally be hyphenated:

This sunflower sea star has a three-foot-wide arm span.

Caption 16, America's National Parks: Olympic

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This seventy-three-mile-long stretch of wilderness...

Caption 5, America's National Parks: Olympic

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When there is an adverb ending in -ly involved, the compound adjective is not hyphenated:

The next step is to put all the potatoes in a lightly greased roasting tray.

Caption 62, Michele from Down Under: Crispy and Spicy Roast Potatoes

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California sea lions are highly intelligent creatures.

Caption 59, Evolution: The Coasts

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Further Learning
You'll find many examples like this on Yabla English. When you see one, note the position of the compound adjective in the sentence, and its components.