The stream inspector
A video file is not a sequence of pictures; it is a sequence of packets, most of which only describe change. Drop one in and look at what your player actually reads: every packet to scale, keyframes standing tall, and a preview that seeks the way seeking really works.
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How to read the timeline
Each bar is one encoded packet, roughly one frame. The amber bars are keyframes: complete pictures that can be decoded from nothing. The dimmer bars are delta frames, which only encode what changed since earlier frames, and that difference in job is the entire size gap you see. On typical footage a keyframe costs ten to fifty times as many bytes as its neighbours.
A run of one keyframe plus its deltas is a GOP, a group of pictures. Encoders place keyframes on an interval (and often at scene cuts, where a delta would cost nearly as much as a full picture anyway), so the timeline is also a map of where your player can start.
Why seeks snap
Click a delta bar and watch the preview: playback cannot begin on a frame that only describes change, so the player jumps to the previous keyframe and decodes forward to reach the moment you asked for. Long GOPs make files smaller and that detour longer, which is why streaming services keep keyframes close (about two seconds) while an archival encode can afford to stretch them out. The keyframes and GOP article goes deeper, and the trim page shows the same fact from FFmpeg's side: stream copies can only cut where amber bars exist.
The bitrate you can see
Bar heights are the rate control's diary. A CRF or quantizer encode spends freely on busy scenes and almost nothing on stills, so heights swing with the content; a constant-bitrate encode holds them artificially level. Spikes at scene changes, tall packets during motion, near-empty ones when the picture holds still: that is the codec doing its job, in a chart nobody usually gets to see.