Adding alt text to images, figures, and tables will help people using screen readers understand what these objects are.

Word: To add alt text to an object, right click the object, select "Edit Alt Text" and enter you a description for your object text.

To add alt text to a table, right-click the table, chose "Table Properties" and select the Alt Text tab. Enter a description for the table.

For more information, see this page on using alt text provided by Microsoft, and this page on how to write effective alt text. 

Adobe: See this alternate text tutorial link to add alternate text directly in Adobe.