An adverbial phrase is a group of related words which play the role of an adverb. Like all phrases, an adverbial phrase does not include a subject and a verb.
Tony decided to move to Reading yesterday. (normal adverb)
Tony decided to move to Slough in June last year. (adverbial phrase)
When an adverbial phrase is at the start of a sentence, it is usual to follow it with a comma.
On the count of 3, leap across.
Glossary of grammatical terms