A phrasal verb is a verb with a preposition or adverb used to add meaning to the main verb.
We had to zigzag through a field full of gopher holes. (with a preposition)
We knew he was too drunk to drive as we watched him zigzag awkwardly toward the door. (with an adverb)
My thoughts ran zigzag, all over the place. (the adverb 'zigzag' modifies the verb 'ran')
The word 'zigzag' is also a noun and an adjective.