“3-D” is just another way of writing “3D”, and it stands for three-dimensional.

It refers to something that has:

• Length
• Width
• Depth

Examples:

• 3-D movies: films that appear to have depth
• 3-D graphics: computer-generated objects with depth
• 3-D objects: real-world solid shapes like cubes or spheres

In short, 3-D = three-dimensional (same meaning as 3D), just a different spelling style with a hyphen.