The ground state describes the lowest possible energy that an atom can have.
Atoms may occupy different energy states. The energy states are discrete, i.e. they occur at specific values only. Therefore an atom can only move to a new energy level if it absorbs or emits an amount of energy that exactly corresponds to the difference between two energy levels.
The lowest possible energy level that the atom can occupy is called the ground state. This is the energy state that would be considered normal for the atom.