You can choose to apply an effect on any object on your scene, which will be triggered the first time it appears on your page.
You will find the transition options in your element properties panel, once you've selected an element.
Effects include :
simple opacity change (without movement)
left>right, right>left, up>down, down>up movement
By clicking on the cog-shaped icon you'll be able to customize each of these transition options : distance from the ending position, duration (in milliseconds) and delay (milliseconds).
Note : You will not be able to use the timeline for an object on which you previously applied a transition effect. You'll notice that in the timeline layer stack all objects with a transition effect are colored in light grey.