Replacement for IP cores
IP cores or Intellectual Property cores are reusable logic blocks which are programmable in the integrated circuits. And these are owned by only single party or company. Generally, these are used in Application Specific Integrated Circuits(ASIC) designs and Field Programmable Gate arrays. So, FPGA's or ASIC building can be build by certain companies.
Where as open cores are the replacement for these IP cores which are open to everyone.All these cores are provided under free and open source licenses and are available to everyone.