I've never heard this term, but I can give it a try.

Most Western musical pieces are built around a certain set of notes known as a scale.  For example, the "C" scale consists of the notes C,D,E,F,G,A,B--that is, the white keys of a piano.  To oversimplify greatly, if you play a more or less random series of these notes, you will get a melody that is more or less pleasing, especially if you begin and end with the root (or "tonic") note of C.

If you start off with the C scale, and then begin introducing notes played on the black keys, such as C-sharp, G-flat, etc., you're going to get something much more exotic, and probably not very pleasing. 

Perhaps that is what is meant by "strange" notes: notes that do not "belong" in the scale that is being used.

(Of course, if you know what you're doing, those "strange" notes can turn "Mary had a little lamb" into something with real spice.)