Metcon? Good question! I don't really have a great definition for it. It's a conditioning workout that stresses specific energy pathways. It can be a combination of many different modalities.
Stuff like this:
Complete as many rounds in 20 minutes as possible.
1 minute of jumping rope
30 seconds of 32kg kettlebell swings
30 seconds of bodyweight squats
Or (classic Crossfit-style)
400 meter run
3 rounds for time.