It is important to indicate the headers in your tables for viewers and screen readers.
Click inside the table so that the Table Tools option will be visible. In Windows, it will default to the Design sub tab, if not select it. In Mac, the Table Design tab should be the default, if not, select it.
If your table has a top header row, check the Header Row box. If your table's first column is a header, check the First Column box.
Select a table style with good contrast so it will be easily visible for those viewing the presentation.