How do bubbles form?
A bubble is basically an extremely thin film of soapy water enclosing air that forms a hollow sphere with an iridescent surface.
The surface of water is made up of hundreds of water molecules that stick tightly together to each other because of inter-molecular forces and something that is called surface tension. This helps the water molecules to retain their shape and stay closely knit to one another.
Have you noticed that when soap is spilled over the floor, the floor becomes slippery? Well, in the same way when you add soap to the water molecules, it reduces the surface tension of the water molecules, their tight grasp on each other becomes slippery, allows the surface to stretch into a bubble which is easily formed when air from your mouth passes through.