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.
A better way is to use the MOD and ROW functions along with conditional formatting to create dynamic row shading that will adjust automatically to fit changes to your table of data.
To find out all the details, read the tutorial on Using Conditional Formatting to Add Dynamic Row Shading.