Short answers, pulled from the story.
Britannicus was the biological son of Emperor Claudius and Valeria Messalina, born on the 12th of February AD 41. He was the legitimate heir to the Roman throne until his father's adoption of Agrippina the Younger's son Nero displaced him. His death on Nero's orders removed the last direct Claudian rival to Nero's rule.
The name came from Claudius' conquest of Britain in AD 43, for which the Senate awarded the emperor the honorific Britannicus. Claudius declined to use the name himself and gave it to his son instead, replacing the boy's original surname Germanicus.
Messalina's bigamous marriage to Gaius Silius in AD 48, conducted while Claudius was away from Rome, led to her execution. Her death allowed Agrippina the Younger to marry Claudius in AD 49 and secure the adoption and promotion of her own son Nero over Britannicus.
Britannicus was poisoned by Locusta, a professional poisoner employed by Nero. The poison was added to the cold water used to cool his drink at a dinner, after a servant had already tasted the drink itself. Tacitus records that he lost alike both voice and breath immediately.
Britannicus died between December 54 and the 11th of February AD 55, the day before what would have been his fourteenth birthday. He died just four months after his father Emperor Claudius.
Britannicus is the subject of a 1669 tragedy by the French playwright Jean Racine, which is still performed today. The future Emperor Titus Vespasianus later erected a gold statue of Britannicus and issued coins in his memory. He was also portrayed by Graham Seed in the 1976 television series I, Claudius.