Conditional formatting allows you to add special formatting options to specific areas of a worksheet.
One of these options is to shade rows of data in a worksheet.
Shading rows makes it easier to read across long rows of data.
This could be done manually for a small data table; but it would be a real chore if attempted on a large one.
To find out all the details, read the tutorial on Using Conditional Formatting to Add Dynamic Row Shading.