Rows are the various column layouts that can be placed inside of sections. It will support all of your modules, background, and is customizable by its length and width.
To add a row:
Hover the green + to add a new row,
Choose the number of columns you’d like to have for the row.
You can then click the gray + to add the specific module in the right column.
The Row Settings
You can add background images or colors to the entire row. By default, rows have a transparent background color.
You can also edit the size of the row.
For more information: https://www.elegantthemes.com/documentation/divi/rows/