A type of phrase that provides extra information - usually about time, place or manner - in a clause. A sentence or clause can contain several adverbials (which, unusually for an English syntax, can be located in various places). Adverbials are usually 'optional' elements in a clause - its central meaning being reasonably unaffected if they are left out.
Twice a day ADVERBIAL time during the Summer ADVERBIAL time I exercise SUBJECT+VERB furiously ADVERBIAL manner in the gym ADVERBIAL place on the rowing machine ADVERBIAL place