a - grid-row property specifies where a grid item starts and ends within the grid It can be used when you want control over how many rows the grid item occupies or precisely place it within the layout. values → auto(auto placement), integer, span #n when integer / integer → specifies which row starts and finishes grid-row: 1 / 3; → Starts at row 1 and ends at row 3
b - grid-column property specifies where a grid item starts and ends within the grid It can be used when you want control over how many columns the grid item occupies or precisely place it within the layout. values → auto(auto placement), integer, span #n when integer / integer → specifies which column starts and finishes grid-column: 2 / 4; → Starts at column 2 and ends at column 4