I know it from German. They use it quite a lot, almost in the same way we use "a proposito" in Italian (informal), and sounds to me like the English "that reminds me..." . So I cheked, and it can be used in English too with the meaning of "related to". Here two examples taken form the Cambridge online dictionary:
I had a letter from Sally yesterday - apropos (of) which, did you send her that article?
Apropos what you said yesterday, I think you made the right decision.
CIDE apropos (RELATED), a propos adverb [not gradable], preposition formal
used to introduce something which is related to or connected with something that has just been said I had a long letter from my sister yesterday - apropos (of) which, have you heard from yours recently?
Apropos what you said yesterday, I think you were right.
apropos (SUITABLE) adjective formal
suitable in a particular situation or at a particular time I thought his remarks about her father were hardly apropos.
The NEW OXFORD Dictionary
preposition with reference to; concerning: she remarked apropos of the initiative, ‘It's not going to stop the abuse’.
adverb [SENTENCE ADVERB] (apropos of nothing) used to state a speaker's belief that someone's comments or acts are unrelated to any previous discussion or situation: Isabel kept smiling apropos of nothing.
adjective [PREDIC.] very appropriate to a particular situation: the song feels apropos to a midnight jaunt.
ORIGIN mid 17th cent.: from French à propos ‘(with regard) to (this) purpose’.