Every engineer needs to remember that every wire that enters or leaves your system is an antenna. That wire can radiate noise and it can also receive noise, as well as carry it into your electronic system. Antennas work both ways, they can receive and transmit, often at the same time. EMI flters stop the noise.
Electrical noise can disable your system or violate FCC (Federal Communication Commission) or European CE regulations. This noise is called EMI, electromagnetic interference. Making sure your product neither radiates or malfunctions when subject to external noise is called EMC (electromagnetic compatibility). A bad design sufers from susceptibility, a good one has EMI immunity.