A testimonial is like a personal recommendation. If you give a testimonial on behalf of a restaurant, then you really like that restaurant.
If you've seen the word testimony — meaning spoken evidence in court — then you might have a clue to the meaning of testimonial. Like testimonies, a testimonial is evidence or proof of something good: getting an A in math is a testimonial to your math skills. Testimonials can also be a spoken or written endorsement of something: You might write a testimonial about the awesome cupcakes on your favorite bakery's website, for example.