A bi-directional template creates a merged document that can send information back to ACT!.
This means, as an example, that it allows you to send data to Excel (like a regular template), have Excel run some calculations and send data back to ACT!.
For this to work, you need to map cells of the spreadsheet to ACT! fields. This is done in a way similar to the way you insert a field placeholder in the template. The difference is that you need to select Excel->ACT! in the dropdown list at the top of the window.
When a cell is mapped to a field in ACT!, there is no placeholder in the field but you can see a comment when you hover the cell.
NOTE: If you need to remove a bi-directional placeholder, delete the cell it is in.