Argthtjtr

I have been looking into German Universities and have noticed the title, i.e. here,

apl. Prof. Dr.

What does the apl. stand for? I could not find this in google.

That is complicated, and varies. Mostly these are people performing as professors in teaching, without having the actual seat (like tenure) usually associated with the regular title.

English Wikipedia explains:

Außerplanmäßiger Professor – conferred, in some German states, to a Privatdozent who has been in service for several years, without formally being appointed as professor.

But the reality is more colourful, as this is partly Ländersache (a matter of differences between the federal states):

The title „außerplanmäßiger Professor“ ("Extraordinary (~unplanned, no-schedule, associate) Professor": apl. Prof., apl. Professor) was created in 1933-1935. The title can be conferred by universities with the right to award doctorates and habilitations to persons who have obtained a doctorate, who mostly possess the teaching authorisation (venia legendi) on the basis of their teaching qualification (habilitation), and who, in the opinion of their faculty, have also performed outstandingly in research and teaching.

The awarding of the title is regulated by the higher education laws of the Länder and, to some extent, by the statutes of the individual higher education institutions.

The designation is not an official or service designation and is not necessarily linked to an employment or service relationship at a higher education institution.

In some federal states, professors should not be appointed as full professors if they are also full-time professors at a university; in others, on the other hand, members of the middle class who are salaried or habilitated are often appointed as full professors.

They do not have the authority in every federal state to use the academic term "professor" without further additions, but are nevertheless often indistinguishable from full professors for outsiders and students. In some federal states, the continuation of the designation after the passing and termination of the activity is tied to a permit by the competent state authority.

As commentator @Janka put it for one type of application for this construction:

This grade is an invention for the sake of grade senority. Some institutions like university hospitals may require their head staff to be professors though they are busy within the hospital and couldn't do the necessary amount of teaching a normal professor has to do.

As @SomeWindowsUser already noted:

There is no real English translation, as this title only exists in germany.

And @Peter Shor

It's not clear that it has a good translation in English.

What is clear that a real/full Professor is one without any extra abbreviations or qualifiers and that the real/full Prof is of higher rank, prestige and pay.