An antisemitic trope is a very mean way of talking about Jewish people. It's like telling a lie over and over again until people start to believe it.
For example, some people might say that Jewish people are always greedy, which means they only care about money and they'll do anything to get it. This is not true, but some people believe it because they hear it so often.
Another example is that some people say Jewish people control everything, like banks or the government. This is also not true, but people say it to make Jewish people look bad.
Antisemitic tropes are hurtful and they can spread hate towards Jewish people. It's important to always treat people with kindness and respect, no matter what they believe or where they come from.