The most important accomplishment of Queen Isabella occurred when she was Queen of Castile, a position she assumed at age 23. At that time, Castile was one of many independent kingdoms on the Iberian Peninsula. After the death of her father, Henry II, a war of succession ensued between her brothers and other relatives and, after her younger brother died, Isabela emerged as Queen. Her marriage to Ferdinand of Aragon, a neighboring kingdom, had been negotiated by their fathers as a political pact when she was only six years old, although another suggested betrothal intervened before the marriage to Ferdinand was carried out in 1469.
In the late 15th century, the efforts of the Catholic forces to expel the Muslim rulers of Spain had been gaining steam. The fragmentation into numerous small kingdoms, however, did not support the push that would crush Moorish power. The subsequent consolidation of these two kingdoms, along with the birth of two children, paved the way for the unification of a much larger territory.
Once that was achieved, the so-called Catholic monarchs—titles that emphasized the return of Christian rule—expanded naval exploration further west across the Atlantic, including the financing of the voyages of Christopher Columbus.
Interesting question! There are actually several important things Isabella made happen. The one she is most known for in the US is her funding, with her husband Ferdinand, of the voyages of Christopher Columbus. This is hugely important both for the financial effects in Europe and the knowledge gained of others' cultures.
She is also important in the formation of Spain. She was Queen of Castile, Ferdinand the King of Aragon. The joining of their countries by their marriage helped formed modern-day Spain.
Isabella and Ferdinand also enabled the rise of the Inquisition. This bout of religious cruelty and murder also resulted in the expulsion of the Jews from Spain, changing the face of the population living in Spain.
In general, Isabella had a great impact on Spain. Not all her actions had the effects she intended. She was very religious, and thought the Inquisition was according to God's plan.