Memo writing is a step in your research that moves you from analysis to the beginnings of synthesising new ideas that may become your new theories firmly grounded in the data you have collected. NVivo offers a way of storing you memos as you write them. You will notice that you lose the screen showing the findings, so you may wish to copy then paste onto a Notebook or WORD document and then paste back into NVivo. Here is QSR's video to help you:
I used memo-writing in my doctoral work and found this book very useful on developing memo-writing skills to work in tourism knowledge research:
Charmaz, K. (2013) Constructing Grounded Theory: A Practical Guide through Qualitative Analysis, 2nd edition, London, Sage.