Latte art is the visual presentation of the steamed milk on top of the espresso. It's not necessary. But it's nice. And customers love it.
The most basic shape is a heart. Then a rosetta (leaf). Then a tulip. All of them are variations on the same theme: pour the milk through the crema (the brown foam on top of the espresso) to create contrast.
I'm not going to teach you how to pour latte art in a book. It's something you have to practice. But here are the basics:
Pour slowly. The speed of the pour determines the speed of the design.
Start high. Start pouring from high above the cup to get the milk through the crema.
Then go low. Once the cup is half full, lower the pitcher so the spout is almost touching the surface. This lets the design form.
Create contrast. The white milk should contrast with the brown crema. That's where the design comes from.
Practice. Every day. Do it a hundred times. I had a barista who practiced latte art on water and soap for an hour every day. She became the best in the city.