Didactic poems are poems which are instructional or educational poem. It is meant to teach the reader how to do something. In the case of John Masefield's "Laugh and be Merry," the poem is designed to teach the reader why laughter and merriment is important.
For example, the speaker tells readers that "the world is better with a song." This offers readers a suggestion about how to make the world a better place. Readers are also told to make the most of life because it is short. The speaker reminds the reader about God and his place in creating things which mankind can celebrate within.