In order to start thinking about why soap is slippery, let's first think about when soap is slippery. Have you ever noticed that when soap is dry, it's not really slippery at all? It's only when you add water to soap that it becomes so slick and slippy!
The reason dry soap is less slippery than wet soap is because of something called friction. Friction is what happens when you rub one surface against another surface. For example, think about what happens when you pull a chair across a tile floor. It drags a little bit, right? That's because of friction. But think about what would happen if you pulled a chair across a wet tile floor: it would pull more easily. In fact, the chair might even slip a little bit, and you'd need to be careful, because even you might slip too! Can you guess why all of the sudden everything has become so slippery? It's because adding water to the surface (the tile floor) reduces the friction! And it's the same thing with soap!