An adjective always modifies a noun. This is a good fact about adjectives because it neatly describes their use. An adjective cannot be used to describe the way someone is running, but it can be used to describe the shirt he wore or the facial expression he sported while he did so.
Adjectives can be single words (such as "red" or "enormous"), but they don't necessarily have to be. Indeed, a group of words can modify a noun. This is called an adjective clause, and it is important to be aware of them when learning about adjectives. If, for example, you were to write "this house, one of the biggest I've ever seen, is also falling to pieces because of the contractor's casual relationship with the building code," then the adjective would be "one of the biggest I've ever seen." This is an adjective clause, which is a group of words modifying a single noun.
Nouns and verbs are usually fairly concrete. A house is a house, a dog is a dog, and running is running. Adjectives have degrees and provide the nuance that nouns lack. So, you can write "a big house," "a small house" or "an enormous house" in order to convey different degrees of size.
Adjectives should be used sparingly in writing. This is a fact of writing that many people forget in an effort to more-accurately express themselves. "The large, luxuriously decorated, impeccably designed and swankily staffed mansion was a sight to behold" is an example of adjective overuse. You can convey the same thought without exhausting the reader by just writing "the mansion was extremely nice and likely expensive." Adjectives' strength is in their numbers -- the fewer the better.