FM, or Frequency Modulation synthesis, is a technique that creates sounds by using one waveform, the MODULATOR, to alter the frequency of another - the CARRIER. FM synthesis was very popular in the 80s, and is very good at creating metallic, digital, or bell-like tones.
These are the factory patches from the most famous FM synth, the Yamaha DX7. You might hear some familiar sounds.
The most famous FM synth is the Yamaha DX-7, which was pretty much standard issue in the 80s. Nowadays dedicated FM synths are rare, but many bigger synths have FM capabilities.
Native Instruments FM8