These terms are for the different areas and sounds your voice makes. There are muscular changes to make these different sounds or to transition from one to another as well.
Chest Voice is sung lower in your range and is dominant there. If you make a loud and low sound you are in Chest Voice. As you go higher in notes, there’s a point where you will feel a strain to reach notes in this voice (sound) or your sound will lighten or switch to save yourself from straining. You will be going into Head Voice, or Falsetto for men.
Head Voice or Falsetto is sung on higher notes and is dominant there. You can take a fairly high note and sing it softly and you will probably be in Head Voice or Falsetto.