1. Europe should not interfere in the affairs of the Western Hemisphere and that any attempt at interference by a European power would be seen as a threat to the U.S.
Only England, in particular George Canning, supported the Monroe Doctrine. Mostly just a show of nationalism, the doctrine had no major impact until later in the 1800s.
2. If a colony in the western hemisphere gained its independence from a European county, that colony must never be "re-colonized" and must remain independent.
In other words . . . a New World colony which has gained independence may not be recolonized by Europe. (It was written at a time when many South American nations were gaining independence).
3. The U.S. will stay out of the affairs of European nations (in Europe).