A vector that is parallel to the yz plane has its x-component equal to zero. This means that the vector does not have any displacement in the x-direction and only exists in the y and z directions. A generic representation of such a vector can be written as (0, y, z), where y and z can take any real values.
To understand this better, consider the three-dimensional Cartesian coordinate system. The yz plane is defined by the locations where the x-coordinate is zero. Therefore, any vector lying completely within this plane will have no x-component. For example, the vector (0, 3, 4) points to the point (0, 3, 4) in the yz plane, clearly showing that its movement is confined to the y and z directions alone.