What does Acts 22:28 mean?

Hours before, the Roman tribune was probably quietly working, trusting the Jews on the temple mount were worshiping peacefully. That changed when he received word that a mob had formed and was trying to kill a man, practically under the windows of the barracks at the Antonia Fortress. The tribune's quest to figure out why this man—Paul—is so odious to the Jews has failed miserably. Now, he learns this Paul, whom he has had chained, tied up, and nearly flogged, is a Roman citizen.

The Roman government gave citizenship to people born in Roman colonies. This included Philippi and Paul's hometown of Tarsus. The honor encouraged good will and spread Roman culture, as well as rewarded particularly faithful cities. Roman citizens had the right to defense in a trial; the right to appeal a verdict; protection from being beaten, tortured, or scourged unless convicted of a crime; and protection from execution unless found guilty of treason. Paul used this right to squeeze an apology from the magistrates in Philippi who had beaten him and Silas (Acts 16:22–39). He uses it here to avoid a flogging. Later, he will use it to appeal to Caesar when it's evident the governor will not release him from unlawful custody (Acts 25:11).

The tribune is a Roman citizen, too, but he was not born to it. This fact helps date the event as paying for citizenship was common under the reign of Claudius (AD 41—54). The Roman might even have bribed administrators to sneak his name on a list of candidates, which was a common means of getting through the process.