Keyframe thumbnails are a snapshot/ preview of your layer at a specific frame, offering a visual overview what a selected layer looks like at each point in time it is keyframed at.
Enable/ Disable keyframe thumbnails
To enable or disable keyframe thumbnails:
Right-click on them in the timeline
Select Show thumbnails or Hide thumbnails
You can also use a keyboard shortcut to hide/show thumbnails: ⇧Shift + T
