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."