Hi,
In this example from the documentation:
product_code,product_parent_id,size
main_product,,big
first_variant,main_product,big
second_variant,main_product,small
You can see that the "main_product" has two variants, "first_variant" and "second_variant".
These variant are based on the "size" characteristic, their values are "big" and "small".
The main product need to have a default variant value set, so in this example for the "main_product" you have to set a default size, in the example the size used is big.
main_product,,
big