Okay, imagine that Crimea is like a toy that both Russia and Ukraine want to play with. Back in 1954, a guy named Nikita Khrushchev was in charge of the Soviet Union, which was kind of like the boss of both Russia and Ukraine.
Khrushchev decided that Ukraine should get to play with the toy for a while because it was closer to them and it made more sense for them to have it. So he gave the toy to Ukraine as a gift, just like when your mommy or daddy gives you a toy that they think you'll like.
But years later, some people in Crimea decided that they actually wanted to play with the toy with Russia instead of Ukraine. Russia and Ukraine couldn't agree on who should get to play with the toy, so there was a big argument and Russia eventually took the toy back by force, which made Ukraine very upset.
Lots of people still argue about who should get to play with the toy, but for now, it's still being played with by Russia. Remember though, it's important to share your toys with your friends and to not take them back by force if someone else is using them.