The X is meant to symbolize Malcolm's true family name from Africa, which he does not know, since white slave traders violently severed his family.

In his autobiography, Malcolm X writes,

"X" replaced the white slavemaster name of "Little" which some blue-eyed devil named Little had imposed upon my paternal forebears. The receipt of my "X" meant that forever after in the nation of Islam, I would be known as Malcolm X. Mr. Muhammad taught that we would keep this "X" until God Himself returned and gave us a Holy Name from His own mouth.

He speaks about hearing Elijah Muhammad speak about how black men in America were mentally, morally, and spiritually dead because of their mistreatment and brainwashing by white people. One of the things Muhammad discussed that Malcolm X took note of was how black people weren't able to keep their family names. Their names were replaced with the names of the slaveowners who purchased them.

Muhammad believed that the true knowledge of themselves would help lift up black men in America. Malcolm X learned about the Nation of Islam while he was in prison. He converted and, in the process, changed his name. Later in life, he converted from the Nation of Islam to standard Islam. At that point, he began using the name El-Hajj Malik El-Shabazz.

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.