A repeating field is a field that contains multiple values of the defined type - eg. the column is of type integer, but the field contains multiple integer values.

Mainstream relational databases does not support repeating fields, so any table will be in 1NF by default. Some non-relational databases supports repeating fields.

You can simulate a repeating field in a relational database by e.g. having the column be of type string, and then have multiple comma separated values in the field. But strictly speaking this is not a repeating field, since the column type is string, and there is only one string. It is still bad design though!

A text column with a list of {comma, space, ...}-delimited values representing a set, e.g. of tags, is something you occasionally see.