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. The table tools default sub-menu should be Design (Windows) or Table Design (Mac), if not select the Design sub-menu.

Windows
Design sub tab in the Table Tools tab of the ribbon in Windows.
Mac
Table Design on the ribbon in Mac.

If the your table's first row is the headers for each column, check the "Header Row" box in the Design menu. If your table's first column is the headers for each row, check the "First Column" box.

Header row box checked on ribbon.

Style

Select a table style with good contrast so it will be easily visible for those viewing the presentation.

Table styles available on ribbon.