It means an introduction to a profession or organization, made in order that progress may be made later. The initial uses of the term are straightforward literal ones. This may be just describing someone who steps over the threshold of a property, or someone putting a foot in the door in order to prevent it from closing. Starting with the nineteenth century the phrase, which comes from the USA, is used in a figurative sense. It comes from the technique of jamming a foot in the door to prevent it closing, used by door-to-door salesmen and political canvassers.

Übersetzung:

mit einem Fuß drin sein, hineinkommen (in einen Beruf oder ein Unternehmen)

Gegenteil(e)

The opposite of "have a foot in the door" is "not to get into something".

Synonym(e):

You could also say: the thin end of the wedge or to have a first introduction to something.