FPGAs are microprocessors that can be reconfigured on the fly, through programming, to do different things. They can essentially be rewired, to change what the chip does, in ways that once would have required making a new chip.
This changes how probes can be reprogrammed in response to discoveries when they reach their destinations, or to work around damage to a probe, or to assign new missions once the primary missions are over, since probes often survive beyond the mission lifetime. ESA has some papers examining these possibilities listed here.
Have FPGAs already been used in this way? What possibilities do they open up for the future? What needs to be borne in mind?