A prefix is a group of letters added to the beginning of a word to modify its meaning. The original word may be recognizable, which is the case in the following examples: 

"Unhappy" means "not happy."

Caption 20, The Alphabet: The Letter U

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The prefix "inter-" means "between" or "among," so "international" refers to being between different nations. "Neo-" means "new," while "proto-" means "first" or "original." 

It was four men from Jamaica who made international headlines.

Caption 5, 16x9 - Cool Runnings: Truth Behind Original Jamaican Bobsled Team

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The palace was rebuilt in a neo-Gothic style.

Caption 45, London: City Sights

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We need some sort of 3D prototype.

Caption 29, Apple Advertising: The Underdogs

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In this sentence, "hyper-" means "extra" or "extreme," and "pan" means "all." 

Many millions of years ago, a race of hyperintelligent, pandimensional beings...

Caption 2, The Hitchhiker's Guide to the Galaxy: The Answer to Life, Universe and Everything

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You may also encounter words with roots that you don't recognize. Here, the prefix "omni-" means "all" and "vore" comes from the Latin word "vorare," which relates to eating or devouring. The prefix "dys" means "bad" or "abnormal," and "lexia" comes from the Greek word for "word." 

Like all bears, the glacier bears are also omnivores.

Caption 60, Nature & Wildlife: Search for the Ghost Bear

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If dyslexia takes over, you are lost in the middle of the ocean.

Caption 24, Movie Trailers: True Spirit

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You may be wondering why some prefixes are followed by a hyphen and others aren't. Generally, a hyphen is used before proper nouns or terms (like "Gothic" above), numbers, or abbreviations. It is also used when the prefix ends with the same vowel that is at the beginning of the main word. Certain prefixes, such as "ex-," "self-," "all-," and "quasi-," are always hyphenated. 

Further Learning
From "anti-" to "super-," you'll find many prefixes on Yabla English.