It applies only to red meat(like cow, sheep, goat, deer, bull, ox, etc) and poultry(like chicken, turkey, duck, etc); and not to fish, milk, cheeses, fruits/vegetables, or other food items.
The concern is either a financial one in that since kosher meat is harder to come by and process, that it may have been switched for a cheaper, non-kosher version; or, a circumstantial one where ravens or rodents may have switched it. Even in a place where only kosher meat is available or they cost the same, the rule still applies mi'd'rabanan. See Shulchan Aruch Yoreh Deah 63 for more information.
Regarding simanim for kosher foods that were sent in the hands of a non-Jew; lechatchila we need two for meat, chicken, and wine, and one for fish. B'diavad, one may accept them with one sign and no suspicion of tampering(in cases of need).
See here for some more(but not inclusive) information:http://www.shemayisrael.com/parsha/halacha/Issue6.pdf.
For a practical ruling, please contact your local orthodox rabbi.