When someone talks about space notes, they are referring to the notes in between the lines on the music staff. There should not be any line going through the note. If there is, then these are called line notes.
The notes in the spaces have specific names in different clefs. We will just mention treble clef and bass clef since that is what most people read.
In treble clef, the spaces are F-A-C-E.
For bass clef, the spaces are A-C-E-G.
Notes found in the spaces in between ledger lines are also considered space notes. Yes, they can be tricky to read, but they are just as important as the notes on the staff.
You may have already figured out what space notes are by now. Even so, it is important to make sure you understand every aspect of reading music, no matter how small the detail. If anything, it must feel good to confirm something you already know.