A custom field is the columns of your contact lists, you can create and customize these columns based on your requirements.
With any new list, the system will create 3 custom fields i.e Email, First name, and Last name.
Email column is mandatory by default since email is necessary for mailing (of-course).
And other fields you can make them mandatory or non-mandatory or create new custom field such as Phone Number, City, Country, ZIP, etc.
What is TAG
TAG in the custom field is the exact name which should match with your CSV list column name while uploading, also make it will match with your contact list.