The session_id is the id of the session of the user. When users are not logged in on the website, the cart of the user is attached to his session id. When the user logs in, the session id field is emptied and the cart_user_id field is set with the id of the user. In your case, you could set the cart_user_id directly and ignore the session id field.
The cart_modified and cart_product_modified fields contain the timestamp of the modification date. In the config, you can define a cart retaining period after which the carts are deleted automatically.