By clicking on the sun, the program automatically places a lens flare there. On the right hand side, you can change detailed settings of this lens flare, such as the opacity of the white veil that appears when looking towards the light source and the distance, which refers to the size of the radius where this effect appears (i.e. Does it only appear when I look right into the sun or does it already start a little further away / while panning towards the sun).
Click preview in order to check whether you like the effect you placed. Should you have placed lens flares, where you don´t want them (e.g. no light source), simply click on them and press delete on your keyboard.
This is how a typical lens flare effect looks like in the final virtual tour (for the entire effect, you need to see this effect in moving images, this merely shows the reflections