For example, 'slow' is an adjective. You can describe something as slow, e.g. 'it is a slow bus'. If you want to use 'slow' as an adverb to describe a verb, you would change it to 'slowly', e.g. 'he walks slowly'.
Adding 'ly' to the the end of an adjective is a common way for the English Language to turn an adjective into an adverb. It is not the only way to form an adverb but it is the most common.