https://en.wikipedia.org/wiki/First_normal_form
https://en.wikipedia.org/wiki/Second_normal_form
A relation is in first normal form if and only if the domain of each attribute contains only atomic (indivisible) values, and the value of each attribute contains only a single value from that domain.[1]
Put simply, a table is in 2NF if it is in 1NF and every non-prime attribute of the table is dependent on the whole of every candidate key.