In Unity (and most engines), cameras can be set up to use Perspective or Orthographic projection.

Perspective Projection will show content in ‘3D’ and will allow content to appear to move into or out of the screen in the Z axis. The size of content will reduce as it moves into the distance which gives this type of camera a 3D feel. The further along the Z axis, the smaller the object will be on screen

Orthographic Projection shows all content without scaling in the Z axis. This means that something that’s at Z=0 appears at the same scale as something that’s at Z=100.

Ortho is often used for UI overlays and ‘flat’ content

Many games will use both Perspective and Ortho cameras. The Perspective will show the content of the game in 3D while the Ortho is used to render the flat UI overlays in front of the game