"Useful idiot" is a term used to describe someone who is being used without realizing it. Imagine your friend asking you to help them with something, but they don't really tell you what you're helping them with. They just tell you it's really important, and you trust them because they're your friend. You might help them out without knowing that what they're doing is actually harmful or wrong.
In politics, a "useful idiot" is someone who supports a cause or a leader without fully understanding what it is they're supporting. Maybe they like the leader because they seem charismatic or they make promises that sound good, but they don't realize that the leader's true intentions are harmful or selfish. So, the leader is able to use this person's support to further their own agenda.
It's important to be careful and do research before supporting anyone or anything, especially in politics. Don't be a "useful idiot" - make sure you understand what you're supporting and why.