The Gemini missions was NASA's human spaceflight program in the early 1960s. The Gemini missions came after the Mercury missions, and before the Apollo missions. Each Gemini spacecraft could carry two astronauts, and 10 spacecraft were launched during the Space Race.
The Space Race was a competition during the Cold War, between the USSR (Russia) and the USA. Both countries wanted to be the best at spaceflight. The USSR were the first country to send a human into space, but the Gemini missions put the USA back in the lead.
Gemini's goal was to develop the technology, so that the later Apollo missions could successfully land humans on the Moon. The Gemini spacecraft practiced the movements that the Apollo spacecraft would use, including orbiting the Moon and docking another spacecraft.
The first two Gemini missions were launched without any passengers, to test the systems and the heat shield. The name Gemini was chosen as it means 'twins' or 'double', and each later spacecraft carried two astronauts. Gemini 12, the final Gemini mission, was manned by James A. Lovell and Buzz Aldrin - who went on to land on the Moon with Neil Armstrong in Apollo 11.