Short answers, pulled from the story.
Briseis was a captive woman from the city of Lyrnessus who became property given directly to Achilles as compensation for his military service. Her husband died at the feet of Achilles during the burning of her city, and she lost all choice in this transaction.
Achilles refused to fight again because his status had been publicly stripped away when Agamemnon demanded she be surrendered to him as payment for losing his own slave Chryseis. His withdrawal from battle caused thousands of Greek soldiers to die before he returned to the field.
Homer's original text gives no physical description at all, but Byzantine poet John Tzetzes described her as tall and white-skinned with black curly hair around the 12th century AD. Dares Phrygius wrote that she was small and blonde with soft yellow hair joined above lovely eyes in his History of the Fall of Troy from the 5th century AD.
Medieval romances transformed Briseis into Briseida starting with the Roman de Troie written by Benoît de Sainte-Maure where she became the daughter of Calchas instead of Briseus. Chaucer renamed her Criseyde in his poem Troilus and Criseyde completed around 1380 while Shakespeare adapted this figure as Cressida in his play Troilus and Cressida performed during the early 1600s.
Pat Barker published The Silence of the Girls in 2018 to center the narrative on captive women like Briseis who suffered most during the conflict. This novel retells the Trojan War from the perspective of those who remain silent or invisible in Homer's original epic.