When Uranium-235 (a specific isotope* of Uranium) decays, it goes through several phases in which it transforms into other elements. The final result is the stable lead-207. In this chain-reaction of radioactive decay, Protactinium comes before Actinium. Hence the name.

https://en.wikipedia.org/wiki/Decay_chain#Actinium_series

*Isotopes are variantions of elements that differ only in the number of neutrons.