What is foundry-specific IP?
It means specific to that Foundry and also specific to their IC manufacturing process.

You can buy IP at ARM, make a design with it and have a Foundry make your chips. You'd be using non-foundry specific IP since it is ARM and your IP. You can go to any Foundry to have your chips fabricated.

But perhaps you need a ROM or RAM to go with your design. Usually you would not use a generic ROM or RAM design as you want your ROM or RAM to have the smallest size possible so ROM or RAM are very process specific IPs. They're provided / generated by the Foundry.

You could not use that ROM or RAM design on a different process or different Foundry as that IP is specific for one process only.