A first lien is a debt that takes precedence over other liens in the event of default. If an asset with multiple liens is sold, first lien holders are first to receive payment from the profits of the sale of any debts payable.
Example: Ken and Kim owe $200,000 on their mortgage. They take out a $50,000 home equity line of credit (HELOC) to make some renovations. If they were to default on their primary loan and fall into foreclosure, the first lien holder would take priority over the HELOC.