This option locks the selling prices of the selected catalogue and saves them into the Individual Prices screen. The prices will no longer update with future catalogue updates (unless you reset the frozen setting).
In the Supplier List > select a supplier > click the Catalogues button and right-click a manufacturer's name and click Freeze Catalogue Selling Prices.
The prices remain frozen even after future catalogue loading, unless you return and use the other right-click option to Reset Catalogue Frozen/Own Prices.