What does a red circle with a white horizontal bar mean?

A red circle with a white horizontal bar means no entry.

This prohibition sign tells road users that they must not enter the roadway, lane, or area from that direction. It is commonly used at the entrance to a one-way street to prevent traffic from traveling against the permitted flow. The sign is aimed at vehicles approaching the restricted entrance, rather than at traffic already traveling in the legal direction.

“No entry” and “do not enter” are closely related phrases, and many road authorities use the latter wording when describing the same sign. That wording overlap can make quizzes confusing, but “no entry” is the standard concise name for the symbol in many international sign systems.

The design is not universal in every detail. Many countries use the red disk and white bar, while some other restrictions use red borders, diagonal bars, or additional pictograms. A no-entry sign also differs from a no-through-road or road-closed sign: it prohibits entering from a particular approach, not necessarily all movement on the road.