Con has two main meanings:

1. A trick or swindle where someone deceives you to get your money or trust. A con artist (also called a conman) might pretend to be someone they're not, tell elaborate lies, or create fake situations to steal from people. For example, someone running a con might claim they found a wallet full of cash and offer to split it with you if you give them money first as a sign of “good faith.” The wallet turns out to be empty, and they disappear with your money. Famous cons include selling fake medicine, pretending to be a prince who needs help transferring money, or claiming you've won a prize but need to pay fees to collect it.
2. Short for convict, meaning a prisoner or someone convicted of a crime. An ex-con is someone who has been released from prison. You might see this in news stories: “The ex-con had turned his life around and now mentors young people.”

The verb form means to trick someone: “He conned me into trading my best baseball card for a worthless one.” When someone has been conned, they've been fooled by a deliberate deception. Being conned feels worse than making a simple mistake because someone purposely took advantage of your trust.