For example, the simple adjective "tall" can be inflected with superlative -est to make its comparative form "tallest".
Note that this only applies to gradable modifiers; it is illogical for absolute modifiers to have comparative forms, though real life is messier than logic. Note too, that this suffix can only be inflected onto shorter words generally of one or two syllables; longer words cannot have this added and are therefore defective adjectives. We use the adverb "most" instead.
There are sometimes minor spelling changes, particularly the doubling of consonant letters, e.g. "big" -> "biggest".
Superlative -est is strongly related to comparative -er.
-est tends to be pronounced /ɪst/ - the final /t/ can be reduced towards a glottal stop /ɪsʔ/.