Markers? If you're thinking sharpies, you're a bit off. Markers are part of the onion-skinning function, and let you control how many frames before and after the current frame you'll see in onion-skin overlay. If you look at the bottom of the timeline, there are several tiny buttons; the fifth says "Modify Onion Markers" when you hover over it. Clicking it opens a menu; selecting "Always Show Markers" will make them visible. You'll see brackets on the timeline, to either side of the frame number. These brackets enclose the area currently visible around the selected frame. You can use your mous to grab the dots on each bracket; dragging them lets you increase or decrease the frames visible around the selected one. The range is customizable for each frame; when you move to the next frame, it resets to default.