To ordain is to make someone a minister, priest, monk, or other member of the clergy. In the Catholic church, for example, a bishop ordains new priests.
When you say that people have been ordained, you usually mean that they've been invested with special religion-related powers. In many Buddhist traditions, senior monks ordain new monks and, increasingly, female monks (or nuns) as well. Occasionally, this chiefly religious verb is used to mean "officially declare" or "decree" in a secular matter, as when a court ordains desegregation.