The 3D grid shows the ground plane of the 3D world. The ground plane is, as the name states, a plane attached to the ground of the scene, where Y equals 0. The ground plane represents the dividing line between up and down, that is, between positive Y values and negative Y values. It is centered on 0, 0, 0.
Click the View pop-up menu above the Canvas, then choose Show 3D Overlays and 3D Grid.
A checkmark next to an overlay type in the pop-up menu indicates that the overlay is enabled in the Canvas.