'Twas is a contraction of the phrase 'It was, ' which is commonly used in poetry to begin a story or poem. An opener poem is a type of poem that is used to introduce a larger work, such as a book or collection of poems. It is usually written in the same style as the rest of the work and often contains themes or ideas that are found throughout the larger work. Openers can also be used to set the tone for the rest of the work.