Traditionally, the glass is flat and all the points of the glass are on the same plane. 2.5D Arc refers to a slight curvature at the edge of the glass display and offers a smoother touch feeling. Smartphones with 2.5D Arc technique improve a lot in visual and solid quality. So it is used on many high-and-mid-end smartphones in 2015 and will be a mainstream trend in upcoming 2016 smartphones.
How does 2.5D Arc glass display look different with common flat glass? This image may show you an intuitive difference among the general 2D (flat) glass, 2.5D Arc glass and 3D glass.