When you import products using CSV in Magento 2 by the native functionality of Magento from System -> Data Transfer -> Import
Select Entity Type -> Products
Import Behavior dropdown display Add/Update, Replace, Delete options.
(1)Add/Update Product: Product are added into admin catalog -> product.from CSV file.
In This scenario, if product is already exist, at that time content are updated for that product and if product is not already available in catalog, create new product.
(2)Replace: Replace works as remove same old product and generate new product with new id.
If you have already a product with SKU ABC and its product id is 1, and you can choose Replace from the dropdown at import time, new Product ABC are created with new id 2 so remove same product with id 1 and generate the new product with new id.
(3)Delete: Remove all product from catalog, match with given SKU from a csv file.