Each row contains one value, of the appropriate type, for each column, and there are no duplicate rows.
This is a simple rewording of the formal definition:
“Let relation r have attributes A1, …, An, of types T1, …, Tn, respectively. Then r is in first normal form (1NF) if and only if, for all tuples t appearing in r, the value of attribute Ai in t is of type Ti (i= 1, …, n).”
Date, C.J. Database Design & Relational Theory. Sebastopol: O,Reilly Media, Inc., 2012. Print. (Emphasis theirs)