An electron is a subatomic particle that orbits around the nucleus of an atom. There is usually the same number as the number of protons; if not then the atom is known as an ion. Electrons have a mass of almost nothing compared to protons and neutrons, and have a negative charge. The antimatter equivalent of an electron is a positron.
A message communicated to Counselor Troi was the phase: "One moon circles." Upon further investigation, with the assistance of Data, they were able to determine that that phrase was in reference to the hydrogen atom, where "one electron circles one proton." (TNG: "Night Terrors")