A Scene is what is displayed in the viewport (what you see on the page ).
A scene displays your elements ( what you see on the page).
You can have many scenes and each scene contains it's own elements.
You normally transition into another scene to change what the person viewing sees in big gulps.
I.e click answer 1 you are taken to scene 2, click answer 2, your are take to scene 3.
Scene 2 & 3 will show the user two different things and allows you to transition without having to re animate all your elements if you only could have a single scene. Think of scenes like jumping from one page to another but within the same viewing area (viewport)
A Timeline is where you can control an elements properties that make it animate.
You can control most elements one way or another on a timeline.
A single timeline can control many elements.
Each scene has it's own timelines. And can have many timelines controlling different elements.
All timelines start from 0 seconds. But you can place keyframes or actions at any point in a timelines running time.
An individual timelines can be started at anytime you want but will always tick away from 0 seconds onwards and fire keyframes and actions set at the time relative within itself.
I think you need to look at the documentation and also have a look at some of the tutorials videos. on Scenes and Timelines and then get a Handle on Symbols.