Wiki does a better job of explaining it here:
http://en.wikipedia.org/wiki/Serotonin_syndrome
Basically its an overload of serotonin in your brain caused by a combination of drugs that increase synaptic serotonin levels. For example, taking Trazodone by itself increases synaptic serotonin levels as does Cymbalta. Taking these drugs by themselves isn't a problem, but combining them (and/or other drugs with similar serotonin level increasing effects) causes too much serotonin to be hanging out in the synapse (the gap between two neurons where neurotransmitters move about, sending messages from neuron to neuron to put it simply).
I've had serotonin syndrome, and it is indeed life threatening and feel a lot like an panic attack, increased heart rate/blood pressure, fever with sweating and chills, paranoia, hallucinations, panic/fear/anxiety and extreme nausea and vomiting, among many many other symptoms (most of which are also
listed in the wiki article).
If you experience these, seek medical help ASAP. It's not something you can handle at home with OTC drugs or prescription drugs you may have, etc and you'll require professional medical care and monitoring.