Okay kiddo, let me explain foreign involvement in the Spanish Civil War as simply as possible.
Picture this, there was a war in Spain in 1936. It was a big fight between the Spanish government, who were called Republicans, and some other people who wanted to take over Spain, who were called Nationalists.
Now, different countries around the world heard about this war and got involved. For example, Germany and Italy sent soldiers and weapons to help the Nationalists, while countries like the Soviet Union and Mexico helped the Republicans.
Some people thought it was a good idea to support one side or the other because they wanted to spread their own ideas or make friends with the winner. Others just wanted to make sure the war didn't spread to their own country.
Unfortunately, all of this foreign involvement made the war much worse and many people died. Eventually, the Nationalists won and ruled Spain for a long time.
But the important thing to remember is that wars are never good, and it's always better if we can find ways to talk and solve problems without fighting.