Those properties define the number of columns and rows in the grid, and they also set the width of each column/row.
The following snippet defines a grid with 4 columns each 200px wide, and 2 rows with a 300px height each.
.container {
display: grid;
grid-template-columns: 200px 200px 200px 200px;
grid-template-rows: 300px 300px;
}
Here’s another example of a grid with 2 columns and 2 rows:
.container {
display: grid;
grid-template-columns: 200px 200px;
grid-template-rows: 100px 100px;
}