ELI5, hmmm? Okay, I'll take a stab at it. For some types of processing, it's vastly more efficient to implement specially-designed digital circuits to do the

ELI5, hmmm? Okay, I'll take a stab at it. For some types of processing, it's vastly more efficient to implement specially-designed digital circuits to do the work instead of using a regular CPU. If the need is high-volume enough, these can be fabricated on custom silicon chips. Common examples are DSPs, GPUs, and custom Bitcoin mining chips. For low-volume applications where it's not cost efficient to fabricate custom chips, there's a specialized type of "generic" chip known as a field-programmable gate array or FPGA. These FPGAs contain a grid of digital logic gates (to oversimplify a bit) and programmable interconnections that allow them to be configured to create any type of digital circuit. While it can't run as fast as a fully custom silicon chip, it's still fast enough to get a tremendous speedup where a custom digital circuit design is beneficial. Now that Amazon has made EC2 instances with FPGA accelerator cards available, ordinary users with the need for these custom digital circuits now have access to these specialized devices without having to make the enormous upfront cash investment of purchasing and operating servers with these FPGA accelerator cards themselves. Addendum: It's also worth noting that, as Moore's law slowly grinds to a halt, that building custom digital circuits using FPGAs for specific processing needs is one of the few promising ways remaining for getting a big boost in compute-intensive app performance in the future. By making these available to a wide audience, Amazon is effectively accelerating the speed at which this type of technology will make it to ordinary desktop computing.