There will be times when you upload a contact file (csv, xslx) generated by another provider and your columns won't have field names. You'll have the option to:
Some columns do not have field names. What would you like to do?
Select field names.
Continue without selecting field names. Note that the columns without field names will not be imported.
Select field names
In most cases you will click Select a field at the top of the column and select Skip Column.
If you continue without selecting field names those columns will not be imported.