将要 is more formal/written than 快要 (kuài yào, about to) or 就要 (jiù yào, about to). All three express imminent future actions, but 将要 is preferred in formal contexts.
usage
将要 = general future (formal). 快要 = emphasis on 'soon.' 就要 = emphasis on 'right away/imminently.' 要 alone is the most colloquial.
