Okay, so you know how the police help catch bad guys who break the law? Well, the Soviet secret police were like that, but they were also responsible for spying on people and making sure everyone followed the rules the government set.
In the beginning, there was a secret police agency in Soviet Russia called the Cheka. The Cheka was created in 1917 to protect the Communist Revolution from people who did not like it. They were used to get rid of enemies of the government and keep people in line.
Then, in 1922, the Cheka became the GPU. The GPU was in charge of making sure no one talked bad about the government, and they also gathered information on people to make sure they weren't trying to overthrow the government.
After a while, the GPU was renamed again in 1934 to the NKVD. At this point, the Soviet Union was under the leadership of Joseph Stalin, who was not a nice guy. The NKVD was responsible for making sure people didn't disagree with Stalin or the government, and they even did terrible things like torture and kill people who they thought were a threat.
But, in 1941, when the Soviet Union was attacked by Nazi Germany, the NKVD had to help protect their country, and they ended up doing some important spy work during World War II.
In 1954, a new government leader named Nikita Khrushchev came to power, and he didn't like how the secret police were acting. So, he changed the name of the agency to the KGB. The KGB was still a secret police agency, but they had to follow some new rules to make sure they weren't hurting innocent people.
Finally, in 1991, the Soviet Union fell apart, and the KGB was dissolved. Today, each country that was once part of the Soviet Union has its own secret police agency to help keep its citizens safe.