An adjective is a describing word. In the following sentence, the word tall is an adjective.
Josie is tall.
If we want to compare two things, we use the comparative form of the adjective.
Josie is tall, but Jack is taller.
If we are comparing three or more things, we use the superlative form of the adjective.
Josie is tall, but Jack is taller and Jemima is the tallest.
For most words with one or two syllables the rule is to add 'er' to make the comparative form and 'est' to make the superlative form.
If the word has only one syllable and has a short vowel then we usually double the final consonant before adding 'er' or 'est'.
big bigger biggest
If the word ends in the letter 'y' then the 'y' must be changed to 'i' before adding 'er' or 'est'.