What type of word is an adjective? If we want to compare two things, people or whatever, we usually add 'er' to the end of the adjective.
We call 'taller' a comparative adjective because we are comparing.
If we have more than two to compare, we add 'est'.
We call 'tallest' a superlative because he is the most tall.
With longer adjectives, of three syllables, we put 'more' and 'most' infront of the adjective. It would sound strange to say 'horribler' and 'horriblest', so we say 'more horrrible' and 'most horrible'.
The same rule applies for adjectives ending in 'ious', for example: vicious.