The two white triangles you see in the screenshot are called markers. They are used to «denote frames with key points or significant events within an animation». You can read more about them in the manual.
However, they are not responsible for the limited playback loop you see in the video. That is controlled by the Start and End frame numbers you see towards the top-right side of the Timeline editor window. In the screenshot above they are pointed out with green arrows, and set to frames $10$ and $120$ respectively (current frame being $20$). You'll notice that those numbers also correspond to the light-gray area in the timeline, indicating the frame range it's gonna repeat when you press play. These numbers also determine which frames are going to be produced when you render your animation, and are repeated in the Frame Range options panel inside the Properties > Output window.
You might wanna have the render output frame range set to something, but limit your loop's start and end frames to something different in the viewport while working. The button with the clockwatch icon in the screenshot, called Use Preview Range (blue arrow), lets you do that. You can see it in action, along with other features of the Timeline in Blender's official tutorial video about it here: Timeline - Blender 2.80 Fundamentals.