I suppose that you are trying the MyFirstProgram from the B4X Getting started booklet.
When you ran the program, I suppose that haven't done anything on the device:
1. On the device, you need to press on the text box
View attachment 93929to display the keyboard, enter a result and press the O K button.
This message
View attachment 93927 means that you can modify the code while the program is running and waiting for user interaction.
If you modify the code, then the program flow is stopped, and after hitting Ctrl + S the program flow is reactivated.
Look at chapter
Debugging in the B4X IDE booklet, for more explanations.
EDIT: Others were faster than me:
Ctrl + S works also in the Main Activity.
But, if you need to reload a layout or reinitialize something you can click on
View attachment 93934
to rerun the program without needing to stop it and rerun it.