A "not" joke is when we're trying to make fun of something... and what we do is, we make a statement that we pretend is true... but at the end, we say "not," which means it's not true.
See on siis, kui millegi üle nalja heita ...ja see käib nii, et ütleme midagi, mille puhul teeskleme, et see on tõsi, ...aga siis ütleme ""mitte"", mis tähendab, et öeldu ei olnudki tõsi.