Hi!
I’m completely new to Blender. I read the manual but still there are some question marks. I don’t understand the function of the lock symbol next to the layer buttons. What is it good for? 
Thanks in advance.
If you unlock it, each of the windows can be viewed, rotated differently
Ok, that was the hint I needed. Thanks!
If you have several viewports, and they are locked, they will always have the same layers active and the same camera active; this is what is rendered.
If you unlock a viewport, it can have a different camera selected ([CTRL]+[NUM 0] if you didn’t know) and different layers active than the others. The view that is rendered is then the one the mouse was over when you pressed [F12], or the view that the mouse was last over before clicking render.
All the locked viewports then still always match, but the unlocked ones can each be different.
phlib: Thank you for your in-depth answer. My mistake was that I tried to figure out the function of the lock button with only one 3D window.