- C is the number of renderers* being rendered over total number loaded
- F is number of renderers being clipped (i.e. not viewable)
- O is the number of renderers removed through occlusion culling
- E (first line) is the number of renderers skipped
- E (second line) is the number of rendered entities over total entities
- B is the number of entities hidden
- I is the number of individually removed entities (the remainder of B and the numerator of E combined)
- P is the number of particles
- T: All is the total number of entities
- MultiplayerChunkCache is the number of chunks currently loaded into memory
- x and z are your current coordinates along the horizontal plane
- y (feet pos) is the current coordinate along the vertical plane of the floor you're walking on
- y (eyes pos) is the current coordinate along the vertical plane of your eye level (Steve's eyes being 1.620 blocks above the ground)
- f is the orientation of Steve: 0 is west, 1 is north, 2 is east, 3 is south
- lc is the height level of the highest non-air block within the current chunk
- b is the name of the biome you're currently in
- bl is the light level of the block your head is in, from block-based lights like torches
- sl is the level of sunlight/moonlight on the block your head is in (independent of the current time of day)
- rl is the maximum of bl and sl
- ws is your walk speed
- fs is your flying speed
- g is whether your feet are on the ground (true if you are, false otherwise)
The bar graph charts frame rate over time inversely (e.g., higher spikes = lower frame rate). Green plots are when your frame rate is 60 FPS or higher.
The pie graph charts the distribution of CPU time across different parts of Minecraft:
-  root (white) is essentially a sanity check: should always be 100%
- [? probably 1]unspecified (green) is the time spent on things not already enumerated
-  render (blue) is the time spent on rendering the world
-  tick (purple) is the time spent on the game clock (heartbeat)
-  sound (yellow) is the time spent on the sound engine
-  updatelights (salmon) is the time spent on updating the lighting engine (usually at sunset and sunrise)
Pressing the 1-9 keys will show sub-divisions of the numbered slices of the pie chart (in addition to switching items as usual), and pressing 0 will go back up.
- A chunk, if you're not familiar with the term, is a section of the world, equal to 16x128x16 blocks.
- Given the context in which it appears in RenderGlobal.java, I believe "renderers" refers to a single block.
- Under the Minecraft coordinate system, X and Z are the horizontal axes and Y is the vertical axis, which confuses players used to other coordinate systems.