The meaning of a straight ally refers to someone who identifies as heterosexual, as well as the gender they were assigned at birth.
Typically, many straight people are also considered “cis,” which is short for cisgender. This term means essentially the same thing: The individual maintains their birth-given gender.
People in this demographic need to support marginalized or disenfranchised members of the LGBTQ community outwardly.
Since cis people do not face the same discriminatory – and, often, outright hateful – treatment that LGBTQ folks do, they typically have more social influence and agency to advocate on behalf of those who have been alienated and denied fundamental civil rights.