The average kinetic energy of the particles in a substance refers to the mean energy of motion of all the particles combined. In the example of a substance made up of 1,000 particles, the average kinetic energy would be calculated by taking the sum of the kinetic energies of all 1,000 particles and dividing that sum by the total number of particles (1,000).
This average value provides a general representation of the energy state of the system. Even though individual particles may have different speeds and therefore different kinetic energies, the average kinetic energy gives us a single value that corresponds to the temperature of the substance. Temperature is thus a measure that reflects the collective motion and energy of particles in a material, rather than the kinetic energy of any single particle. So, to answer your question: the temperature of the substance indicates the average kinetic energy of all the particles within that substance.