A Field-Programmable Gate Array (FPGA) is an integrated circuit device that contains the functional equivalence to a huge number of logic gates, all wrapped into a single package. The unique feature of an FPGA is that it can be programmed to perform virtually any function.
Depending on the used technology an FPGA can be programmed many times, which makes it possible to change the behavior of the device, e.g. to fix a bug. This is a big advantage to traditional digital design, where a change in functionality requires to produce and assemble a new PCB and possibly deliver it to a customer and install it on-site (qualified manual labor is expensive).