This option will open the document in Office Online, the web-based version of Word, Excel, and PowerPoint. Changes that are made to documents while they are open in the browser will be saved virtually as they are made to the document.
Other users will be able to see that the document has been opened by another Actionstep user and will have the option to also edit it. Changes made by both users will appear in real time and can be seen by each user.
See Opening a Document In Microsoft Office Online for more details.