I've been dumb and fearless! It's a personal goal - however, my smash into animation triggered this thing which might be an F Curve somewhere? More importantly than what it is in any detail is getting rid of it. Perhaps useful later, it was unsolicited this time.
You must have accidentally pressed M on the keyboard while your mouse was hovering over the timeline. These are called markers in Blender.
To delete it, select the marker with left-click (LMB), keep your mouse hovered over the marker area, and then press X then Delete Markers.