idem is a Latin term meaning "the same". It is commonly abbreviated as id., which is particularly used in legal citations to denote the previously cited source (compare ibid.). It is also used in academic citations to replace the name of a repeated author.
Id is masculine and neuter; ead. (feminine), is the abbreviation for eadem, which also translates to "the same".
Here, the first citation refers to the case of United States v. Martinez-Fuerte. The volume number cited is 428 and the page on which the case begins is 543, and the page number cited to is 545. The "U.S." between the numerical portions of the citation refers to the United States Reports. 1976 refers to the year that the case was published. The second citation references the first citation and automatically incorporates the same reporter and volume number; however, the page number cited is now 547. Id. refers to the immediately preceding citation, so if the previous citation includes more than one reference, or it is unclear which reference Id. refers to, its usage is inappropriate.
Here, Id. refers to the Executive Order that was mentioned in the previous sentence.
In this example, Id in the second citation indicates that the author is identical to that of the previous citation. That is, the author of the second citation is also Macgillivray, J. A.