When you press the Play button on the upper right-hand corner, you enter a kind of viewing mode. You will be able to look around and adjust the camera perspective – but not change anything about the space. If you previously added sound to your space, it will be played in this mode.
Also, if you added some script (only if you are in the early access program for the scripting API), you will see it being executed in the play mode.
Basically, the Play mode lets you experience your own space like an outsider would see it with the sharing link.
To go back to the building mode, click the button on the upper left-hand corner.