Saikeirei is a polite way of bowing in Japanese culture. Just like when you say "please" or "thank you" to be polite in English, people in Japan bow to show respect and politeness.
When you do a saikeirei, you bend your waist from the hip and keep your back straight. Your hands are placed together in front of your body, with the tips of your fingers touching each other.
This type of bow is usually done in formal situations like meeting someone important, thanking someone for their help, or showing respect to someone who has passed away. It's a way to show that you respect and value the other person.
So, think of it this way: saikeirei is a special way of bending your body and putting your hands together to show others that you are being very polite and respectful.