Row & Colum Keyframe

The tool that looks like a keyframe with a left and right arrow in the timeline column lets you set a keyframe at the current CTI or move to the previous or next keyframe with a click. You can also use the J and K keys to move to the previous or next keyframe. 

The keyframes in the timeline show you where a change has been made property showing in the timeline. In your case, there are keyframes for the Range Selector/Start property because a Text Animator was added to the text layer, and keyframes have been set.