Something considered taboo is naughty, something society considers a no-no. For example, it is taboo to ask people how much money they earn.
Taboo is one of those rare words in the English language that doesn't have Greek or Latin or French roots, coming instead from the South Pacific Islands in the eighteenth century via the explorer Captain Cook. For many islanders some things were tabu, meaning not only could they not be done but they could not be talked about either for risk of offending the Gods. This latter meaning still applies today (minus the spiritual underpinning), when we say that certain subjects are "taboo," meaning off-limits for discussion.