I assume you are asking about the "X" in Malcolm X's name.
The use of "X" as a last name was started by the Nation of Islam. The idea of using the "X" was that African Americans' real last names had been lost because of slavery. Whatever last names they had had in Africa did not come with them.
Instead, many African Americans carried last names given them by their masters. For this reason, some in the Nation of Islam referred to their last names as "slave names." Instead of continuing to use these "slave names" they took the name "X" as a way of rejecting slavery and symbolizing what they had lost when their ancestors were enslaved.