"Fissioned" in any nuclear context means big atoms are split into small atoms.
Natural uranium is 0.7% U235, and the rest U238. Only the U235 is fissionable by the slow neutrons in conventional reactors. For a nuclear plant, we have to enrich the uranium until it's at least 2% U235 (or a little more, depending on reactor design...the top is about 5%).
The U238 in the reactor doesn't fission when hit by a neutron, but may absorb it and turn into plutonium, which is fissionable. About a third of a conventional reactor's energy output comes from fissioning plutonium.
Other transuranics (elements heavier than uranium) are also produced in a conventional reactor as nuclei absorb neutrons without fissioning. The end product is a mixture of U238, unfissioned U235 and plutonium, other transuranics, and fission products. There's some fissionable material left over because some fission products absorb neutrons, poisoning the reaction; some countries, like France, "reprocess" this mixture to pull out the remaining fissionables.
A fast reactor changes all this. In a conventional reactor, neutrons are purposely slowed down by materials like graphite and water. In a fast reactor, the neutrons from fission are left at the high energies they start with. These neutrons can fission U238 and all the transuranics.
So the fast reactor can use all of the uranium, instead of just 1% percent of it (U235 plus some U238 that gets converted to plutonium). That does mean it only needs about 1% as much uranium input.
A 1 GW coal plant uses a 100-car trainload of fuel every three days. A conventional nuclear plant uses an 18-wheeler load of fuel rods every 18 months. But a 1GW fast reactor uses just one ton of fuel per year, about the size of a beach ball. It can supply all the energy you need for your entire life, transportation included, from a piece of fuel the size of a golfball.