Answer: In brief, a modifier is a word or phrase that changes another word in the sentence. For example, one-word modifiers include very, quickly, and frequently.
Question: Does it matter where you put them in a sentence?
Answer: Of course! Modifiers should always be close to the word they change. One-word modifiers generally go before the word they change. Modifier phrases often go after the word they change.
Question: Where can I find more information on modifiers?
Answer: Here are some resources I find helpful.
Grammar Girl offers a quick and easy to understand explanation.
The Guide to Writing and Grammar gives examples (and non-examples) of successfully using modifiers.
Lastly, Purdue's Online Writing Lab (OWL) is always a great resource.