an adverb is an invariable part of speech that modifies a verb, an adjective, another adverb, a phrase, a clause or a sentence. An adverb is generally used to add more information about time, manner, place, degree, etc. Completely is an adverb that denotes how something is done or how the state of something is described without any restriction or limitation.
The use of completely is mainly to emphasize completeness or fullness. It is used to emphasize how something was done in entirety.
1. He completely understands the issue.
2. We ate the cake completely.
3. The room is completely silent.
Care should be taken to use completely in the right context when used for emphasis. For instance, it should not be used when something was partially done as it will render the sentence incorrect. Furthermore, it should be used after the verb, adjective or adverb it modifies.