Within a grid container, there are grid cells. Any cell positioned and sized using the grid-template-* properties forms part of the explicit grid. Any grid cell that is not positioned/sized using this property forms part of the implicit grid instead.
Understanding explicit grids and implicit grids is powerful. This is my quicky take:
Explicit: you define a grid and place items exactly where you want them to go.
Implicit: you define a grid and let items fall into it as they can.