Symbolism, Imagery, Allegory
The narrator of Sir Gawain is very clear about what the pentangle (five-pointed star) on Gawain’s shield represents:
It is a symbol that Solomon designed long ago
As an emblem of fidelity, and justly so;
[...]
Therefore it suits this knight and his shining arms,
For always faithful in five ways, and five times in each case,
Gawain was reputed as virtuous,
(625-626; 631-633)
These five ways in which Gawain is virtuous are in the dexterity of his five fingers, the perfection of his five senses, his devotion to the five wounds of Christ, his reflection on the five joys of Mary in Christ and, finally, five virtues: generosity, fellowship, chastity, courtesy, and charity. Wow, that’s a lot of virtue.
The pentangle is an appropriate representation of these five areas of virtue because each of the five sides of the pentangle transitions seamlessly into the next. This aspect of its geometry might represent the way in which the virtues are interrelated, each area feeding into and supporting the other.