The Molotov-Ribbentrop Pact was an agreement between two countries, Russia (then called the Soviet Union) and Germany, that decided they would not fight each other during World War II. They made this deal in August 1939, just a few months before the war began.
Think of it like when two kids agree to share their toys and promise not to fight over them. In this case, Russia and Germany agreed to not fight each other over land or resources.
However, this agreement was not just about sharing toys, it was also about secretly dividing up territories that they both wanted to control. They made a plan to split Poland between them and other parts of Europe, which was kept secret from the rest of the world.
This agreement lasted for two years until in June 1941, Germany broke it and invaded the Soviet Union. This led to a major conflict between the two countries that lasted until the end of the war in 1945.
Today, many people see the Molotov-Ribbentrop Pact as a very bad thing because it helped pave the way for World War II and allowed for many terrible things to happen.