Enriching uranium refers to a technological process in which U-235 is increased in proportion to U-238. U-235 is an isotope of uranium that can undergo fission. Fission is when a neutron smashes into the nucleus of a uranium atom and splits the nucleus. This splitting then releases other neutrons that smash into other U-235 atoms and releases more neutrons. In order to build a bomb, the uranium has to be about 90% U-235. Iran says it has enriched uranium to about 3.5%. That means that about 3.5% of the isotopes in their sample are U-235. Since in nature it is less than 1%, they have increased it quite a bit. Yet to become a nuclear weapons state, they have to go a lot further than that. In addition they would have to weaponise it and put it on a rocket or in a bomb.
Uranium is a metal that you dig for like gold from underground. It appeared I presume when the Earth was created and is not that plentiful. In any event less than 1% of Uranium is U-235 which again is the most suitable nuclear fuel for an Atomic Bomb. Enrichment refers to increasing the odds that fission can occur on a scale adequate enough to produce electricity or an atomic bomb. Iran claims it can use the centrifuge technique to accomplish this task by spinning the uranium component that drives the heavier U-238 to the containers of the centrifuges. This means they convert uranium into a gas, spin it with centrifugal force in which the somewhat heavier U-238–three neutrons heavier–is whisked to the sides of a centrifuge. If there are enough centrifuges the ratio of U-235 to U-238 will increase.
While terrorist nations like the U.S., which adores the bomb and is the only nation that has used the weapon in combat, seek to prevent Muslim nations from acquiring it, one should know that the A-bomb is obsolete for the U.S. More on that in additional posts as the nuclear-terrorist United States with its ally Israel seek to use nuclear weapons to intimidate and force their colonial will on the Middle East. They are insane in believeing that the capacity to inflict violence and mass death through nuclear escalation dominance is the way to pursue justice and democracy within the international community. The US has just about run its course as a power for good in the world. The game is over and it must be stopped.