Microchips of the past were mostly 2d. Like a printed circuit board. Today and more in the future they are more 3 dimesional, like a circuit board that has circuits at multiple levels. If my memory and understanding is correct, current state of the art flash memory chips are 100 levels deep. Can we make that a 1000? CPU chips aren't taking advantage as much of this multi level architecture. Same with GPU (graphic processor units) or TPU (tensor processing units) chips.

3d comes in multiple flavors. Here is an example of stacking the chips: https://en.wikipedia.org/wiki/Three-dimensional_integrated_circuit

An analogy is cities. Today's cities are mostly 2d. Traffic happens at the ground level. In the future we will have cities were commuting can occur at multiple vertical levels and we can call them 3d cities.