An idiomatic phrase expressing that if something looks and acts a certain recognizable way, it probably is exactly what it appears to be.
"After the liquor store was robbed, a man was found leaving the scene carrying a gun. Did he commit the crime?"
"If it walks like a duck, looks like a duck, and quacks like a duck, it's a duck."
When a guy tells you this it basically means he can eat you all day. It could be used as complement or Sexual term but either way we all know what term you'll choose