Units and Grid Help, Please

I have the units set to Metric. When I zoom in, in the upper left corner it tells me it’s 10 millimeters. Does that mean that each big square of the grid is 10 millimeters or are the little squares inside the big square 10 mm each? Thank you!
Here’s a ref pic.

The smallest currently visible grid squares. You can see from the size of the default cube (2m x 2m x 2m)

Oh jeez!! I should have known this. Thank you Richard for answering.