What are some major English poems produced in later half of fourteenth century?

The most important major English poems produced in the latter half of the fourteenth century are Geoffrey Chaucer's The Canterbury Tales and Sir Gawain and the Green Knight as well as Pearl (which are thought to be by the same poet, known sometimes as "the Gawain Poet" or "the Pearl Poet").
These poems are written in Middle English, a form of English that has enough dissimilar words and grammar to modern English that it can be almost be considered another language. The Canterbury Tales is more accessible to us than the Pearl Poet's works because it is written in the Middle English of London and the south of England, which became the basis of modern English. The Pearl Poet's works are very difficult because they are written in a northern dialect of Middle English.
The three poems mentioned exhibit the breath of creativity in England at the time. The Canterbury Tales are notable for being bawdy, secular tales heavily influenced not only by Christian stories but by Classical lore from Greek and Roman sources, showing the impact of the Renaissance. Chaucer also offers us realistically drawn characters, some of whom, like the Wife of Bath, are unforgettable for their irrepressible energy and verve.
Sir Gawain and the Green Knight is an exemplary romance tale of King Arthur and his knights, representing a genre that was sweeping across Europe at the time. Pearl is an exemplary religious poem, combining the motifs of grief over the loss of a child, a dream vision, and Christian faith in an afterlife.