Ground usually means the common voltage to which all others are referenced (compared). Typically you make things easy for yourself and call that 0V. Often ground will be connected to the enclosure, perhaps to mains ground (and thus to the earth itself), but this is required. Most (not all) circuits choose ground to be the same as the negative terminal of power supply or battery.
Physically you can't just have a single voltage - voltages (more correctly electric potentials) are arbitrary and only the difference in voltage between two signals is physically meaningful. Choosing one common ground for a whole circuit makes things simple.