Often times you will be working in different notebooks (with different kernels) and you will want to share a variable from one notebook to the other.
In order to share a variable (e.g. string, dict, Series, DataFrame, etc.) just use the %store magic:
On the “sending” side:
%store mydf
On the “receiving” side:
%store -r mydf
That's it!
Read more about the %store magic here: