An adjective is a word or group of words that modifies or describes a noun (a little girl) or a pronoun (he is strange). Single-word adjectives usually come before the word they modify (the red wagon). But when adjectives join a preposition to form what's called an adjectival phrase, they can come after the word they modify (the wagon red in color).
Most adjectives come in three states. The positive state describes the basic attribute (the hot plate). The comparative state shows more of the attribute (the hotter plate). The superlative state shows the most of the attribute (the hottest plate).
Sometimes you add “-er” to form the comparative state and “-est” to form the superlative state. But other times you use the word more (more difficult) to form the comparative and most (most difficult) to form the superlative. You'll find a complete discussion of this problem in the section on Adjectives in Parts of Speech on Grammar.com. Click here for the beginning of that discussion.