Adverb
An adverb is a word that describes a verb. They tell the reader how the verb is or was done.
In the phrase swimming confidently 'swimming' is the verb and the adverb confidently describes how the person is swimming. In this case confidently is the adverb.
In the phrase running quickly, 'running' is the verb and the adverb quickly tells us we are running quickly and is the adverb.
In the phrase reading quietly, 'reading' is the verb and the adverb quietly tells us the person reading is being very quiet whilst they read - it is the adverb, telling us how the person is reading.
We can recognise adverbs as they often (but not always) end in -ly and is often paired with the verb it is describing, usually coming just before the verb in a sentence (although can appear anywhere in a sentence as long as it makes sense).