Message box:

This activity is used when the user wants to display or notify the output from certain activities such as extracted data/ bot action. This appears after running the respective activity.

How to use Messagebox? 
1.    Open a project.
2.    Search for Messagebox activity and drag and drop it to the workflow.
3.    Double click on the activity.
4.    Enter the input value within the double quotes or you can enter the string variable.
5.    Give a title to the Message box.

Technical Reference: