A timeline is where you can see all the current steps in your presentation. The timeline shows your steps in chronological order from left to right. If you have multiple steps, you can navigate through your timeline by sliding from left to right.
The steps are the individual slides in your presentation where you can add content. Each step has a thumbnail that represents the step.
To add a new step to your presentation you need to tap on the (+) button, this will add a new step at the end of the timeline.