[apendleton]

Yes, the gist is that in a fusion device, you've got a chain reaction where the splitting uranium or plutonium is making neutrons, and those neutrons strike other uranium or plutonium, which makes more neutrons, etc. But you have a very short period of time to take advantage of this chain reaction, because before long, your device has blown apart, so a large percentage of the fuel in a conventional fission device never undergoes fission (the device has blown up before it has a chance). The job of the fusion reaction in a fission-fusion-fission device (how "modern" thermonuclear weapons work) is, in addition to producing energy of its own, to rapidly produce lots of lots of neutrons, which can very quickly fission a second quantity of fissile material.