Most adjectives and adverbs come in three states. The positive state describes the basic attribute (the hot plate, he ran quickly). The comparative state shows more of the attribute (the hotter plate, he ran more quickly). The superlative state shows the most of the attribute (the hottest plate, he ran most quickly).
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 and Adverbs in Parts of Speech on Grammar.com. Click here for the beginning of that discussion on adjectives and here for adverbs.