Debug mode expands the HUD with extra pieces of information about the state of the simulation as well as the particle currently under the mouse.
There's a lot of information exposed in debug mode. See the image below:
1. Framerate: this shows how many update cycles the game can do in a single second.
2. Particle count: the number of particles currently in the simulation.
3. Particle type: the type of particle that you're currently hovering over.
4. ctype: the "secondary type" or ctype of the particle you're currently hovering over.
5. temp: the temperature of the particle.
6. life: The life value of the particle. Used mostly to store the amount of frames this particle has yet to exist.
7. tmp: An integer that can be different for every particle. Used for various purposes.
8. index: The particle's index in the underlying list of particles.
9. and 10. are the X and Y coordinate of the particle.