The Feynman technique is a learning technique that aims to understand something in depth. This technique was discovered by a Russian and Polish Jewish physicist who immigrated to the United States at the end of the 19th century.
Choose a topic that you want to understand. Try to create a simple concept based on your understanding and then say it as if you were explaining it to a child. Review what you explained. Look for parts you still find challenging to understand, and then look again for more detailed explanations from various sources. Read it repeatedly until you can explain it in your language. Explain it again by expressing it either to a friend or to yourself.
By carrying out these steps, you have implemented the Feynman technique. The understanding you gain from this technique will not only last a day or two, but over a long period.