Watergate salad is a yummy dessert that's made up of different things that taste good when mixed together. The main ingredients are something called pistachio pudding (which is a type of sweet powder you add milk to), whipped cream (which is fluffy and comes in a tub or can), cool whip (which is also like whipped cream but even fluffier and comes in a tub in the frozen aisle), and mini marshmallows (which are like squishy and sweet little pillows).
You also add some other things like canned pineapple (which are small pieces of a sweet fruit), chopped nuts (usually pecans), and sometimes even shredded coconut. All of these things get stirred together until they make a big bowl of sweet, creamy goodness.
It's called "Watergate" salad because it was created during a time when a scandal called the Watergate scandal was happening in the United States, and people thought it was funny to name a salad after it. But don't worry, the salad doesn't have anything to do with the scandal and it's just a yummy treat that people like to make for parties and gatherings.