The Israel Lobby in the United States is a group of people who work together to support Israel and make sure the United States government supports Israel too.
Just like if you have a best friend, you want to help them when they need it, and that's what the Israel Lobby does- they want to make sure Israel gets help from the United States.
They talk to politicians and try to convince them to support Israel, they donate money to political campaigns, and they have big events to show off how great Israel is.
Sometimes people disagree with the Israel Lobby and think that they shouldn't have so much power. But the Israel Lobby thinks they are doing the right thing by helping Israel and making sure the United States supports them too.
So basically the Israel Lobby is like a group of people who want to make sure Israel is happy and supported by the United States, and they work really hard to make that happen.