Isotopes and enrichment
A given element can have different numbers of neutrons. While the protons determine an elements chemical properties, the different number of neutrons can have different nuclear properties. Uranium occurs naturally with mostly u238, with only about 0.07 percent of it occurring as u235. It’s the uranium 235 that can support a nuclear chain reaction. For most nuclear power reactors, you need around 4 or 5 percent of the uranium to be u235. If you want a nuclear explosion, it requires more than 90 percent. The process of increasing the percentage of 235 in uranium is called “Enrichment,” and it is a difficult and dangerous process.
Here are a couple of articles about uranium and enrichment.