Earth is an electric conductor. When a charged body is connected to the earth, it becomes electrically neutral. When a positively charged body is grounded electrons coming from the earth neutralize the body. When a negatively charged body is grounded electrons from the body flow to the earth and the body becomes neutral. The earth is so big that if charge is added or taken away from it its potential does not change at all. Likewise if water is taken away from sea or poured in the water level does not change. The earth is always taking charge from different bodies and simultaneously it supplies charge to other bodies. Hence earth is considered charge less. To determine the height of a place the height of the sea level is taken as zero, similarly to determine the potential of a body, the potential of earth is taken as zero.
Zero, Positive and Negative Potential:
The potential of an uncharged conductor is taken as zero When a charged conductor is connected to the earth its potential becomes zero. Because, in the connected state , both the conductor and the earth is considered as a single conductor. The potential of a positively charged body is positive and negatively charged body is negative.