The baseline is your share of conversions that are not influenced by marketing at all. Those conversions are only happening because of your overall brand awareness and are acquired via organic channels such as organic, direct, and paid search brand.
Of course, the baseline will lose conversions once you stop marketing activities for a longer period of time, but for a short-term period, you can expect this conversion level without running any (paid) marketing campaigns.
In addition to the baseline conversions, there are so-called 'incremental conversions' present. Those are conversions influenced by marketing activities and only appear due to your paid marketing efforts.