A Charlie is a Vietcong soldier, with the name coming from the American soldiers referring to them as Victor-Charlie or just Charlie. To be specific, "Charlie" refers to any
commie force in North Vietnam.
The
meme associated with Charlies is that one of their
war attacks was brutal guerrilla warfare, in which they would hang from or hide around trees/shrubs. Often times before they attacked they would speak to other soldiers hiding, giving signal to open fire. The
meme comes from tense phrase "The trees speak in Vietnamese."