What is parallax method?
If the measurement is made wrong by viewing from a wrong position instead of the correct position leads to an error in the measurement, this error is called parallax error.
Or
The apparent shift between the two objects when the eye is moved sideways is called parallax.
Correction of parallax error:
If two objects are placed at the same distance from the eye, there will be no relative shift between them. If they are at different distances, the nearer object moves in a direction opposite to that of the eye and farther one in the same direction. Once their relative positions are known, they can be brought to one position by suitable shifting. When the two objects occupy the same position in space with respect to the eye, then the apparent shift disappears and it is said that the parallax error has been removed.