an IP core is an intellectual property; a 'building block' you use from another person/firm.
You can download for instance an USB core for your FPGA from opencore.org (or some other websites from core 'manufacturers' where you have to pay for...) and develop a product with it. Like a USB webcam.
It is a block that is already programmed and you can use in your design. You just have to send commands to it an you receive commands from it.
You can image it as an library of functions in C.
I hope this helps.
Maybe you can read this link: **broken link removed**