For example:
* tall -> tallest (the highest degree of tallness)
* happy -> happiest (the highest degree of happiness)
* fast -> fastest (the highest degree of speed)
It is important to note that "-iest" is only used for adjectives that end in "-y". If an adjective doesn't end in "-y", you would use "-est" to form the superlative.
For example:
* big -> biggest
* strong -> strongest
* good -> best
Hope this helps!