Identity labels constantly change because society and identities themselves are in constant flux. “LatinX” is an inclusive, gender-neutral term that refers to people of Spanish-speaking Latin American descent. It is meant to replace Latino/a, or Latin@, but is often used interchangeably with “Hispanic” or “Chicana/o” (though Chicana/o refers only to people of Mexican descent, not all LatinX people).
Most agree that LatinX should be pronounced as if you were saying Latino but–obviously–replacing the last letter with X; i.e., Lahteen X. That said, you’ll probably hear it more often pronounced as Latin (the dead Roman language) X.