Short answers, pulled from the story.
It was a plot in AD 65 to assassinate Emperor Nero, organized around Gaius Calpurnius Piso as the intended replacement. Senators, equestrians, and military figures all joined. It is remembered as the most famous and wide-ranging conspiracy ever aimed at Nero's throne.
A freedman named Milichus betrayed the plot to the emperor on the 19th of April AD 65. The conspirators were arrested almost immediately after.
Nineteen people were put to death and thirteen others were exiled, reflecting the broad scope of the plot.
In AD 40, Caligula took a liking to Piso's wife, Livia Orestilla, forced her to leave Piso, then accused Piso of adultery with her to manufacture a legal basis for banishment. Piso returned to Rome about a year later, after Caligula's assassination.
It is a surviving Latin panegyric, whose title translates as On the Praise of Piso. It takes Gaius Calpurnius Piso as its subject and is one of the works that kept his memory alive in Roman literature after his death.
Tacitus credits Piso with eloquence, generosity, and skill as an advocate, but also identifies significant flaws: a lack of earnestness, excessive ostentation, and an appetite for the sensual. He considered Piso's overall integrity questionable.