I had a lymph node biopsy (neck) that came back atypical cells. What does this mean and how likely is this cancer?

More investigation. Atypical cells means that the pathologist saw cells that weren't definitive for cancer, but were abnormal. These can be due to a number of things (infection, poor fixation, radiation, cancer, etc.), but rendering this diagnosis either means the pathologist will do special tests to determine this or lets your clinician know they may need more tissue if they are still suspicious.