It's a lobby organization in favor of the state of Israel. They spend significant amounts of money on elections, to favor the side that best represents Israels interest.

Edit: thanks for asking the question. So Wikipedia said they were founded after the backlash against Israel for their Qibya massacre. Which is when Israel decided to massacre a small village to make a statement ("kill any Jew and we will kill 20 of yours", I guess). And the world was actually outraged. Also the guy who led the operation would later become the prime minister of Israel. And the government of Israel communicated the obvious lie that they had nothing to with this heinous crime...