The structure of a sonnet:

The Shakespearean sonnet has 14 lines divided into three stanzas of four lines each and a final couplet.

The meaning of Volta:

In a sonnet the Volta is the turn of thought or argument. In petrarchan or Italian sonnets it occurs between the octave and the sestet and sometimes between the 8th and 9th or between the 12th and 13th lines of a Shakespearean sonnet or an English sonnet, before the final couplet.