Okay, kiddo, mistakes were made is something people might say when they did something wrong or made a bad decision. It's kind of like when you accidentally spill your juice all over the floor, and you say "oops, I made a mistake".
But when adults say "mistakes were made", they usually mean that they messed something up, but they don't want to take full responsibility for it. It's like they're blaming the mistake itself, instead of owning up to what they did wrong.
It's important to remember that everyone makes mistakes, and it's okay to mess up sometimes. But it's also important to learn from our mistakes and take responsibility for our actions, instead of just saying "oops, mistakes were made" and hoping nobody notices.