This is a rather unpleasant term that Peter Dutton has borrowed from the far right in America. The term was coined in the U.S in the early 1990s and refers to a child born to an immigrant mother who is illegally in the U.S. without citizenship, the child having American citizenship as a birthright and therefore anchoring the mother and other members of the family in America.
In Australia there is no birthright citizenship as exists in America, so the term is hardly applicable. However, it no doubt works as a general smear, implying that the parents had children for the sole purpose of making it difficult to deport them.
A related term is anchor child, that is, an unaccompanied minor sent by its parents to a foreign country where it will be given refuge. Eventually this will provide the family with a right to enter the country and be reunited with their child.