Its a <u><em>verb </em></u>because biking is an action.
<em>A verb is</em> a word used to describe an action, state, or occurrence, and forming the main part of the predicate of a sentence, such as hear, become, happen.
Examples of Action Verbs in Sentences
Anthony is throwing the football.
She accepted the job offer.
He thought about his stupid mistake in the test.
John visited his friend for a while and then went home.