I mentioned the explicit versus the implicit grid in my last article. The explicit grid is the grid that you create with the grid-template-columns andgrid-template-rows properties. By defining your column and row tracks, you also define lines between those tracks and at the start and end edges of your grid.
Those lines are numbered. The numbering starts from 1 at the start edge in both the block and inline direction. If you are in a horizontal writing mode, with sentences which begin on the left and run towards the right this means that line 1 in the block direction is at the top of the grid, and line 1 in the inline direction is the left-hand line.
Source: https://www.smashingmagazine.com/2020/01/understanding-css-grid-lines/