As a slang feature, it indicates total honesty and directness . To tell someone something "straight up" is to speak without sugarcoating or ambiguity.
If you are looking for specific applications of this term across different contexts, here are the primary "features" it describes: Straight Up
In its most literal and direct sense, a feature refers to a vertical orientation or something positioned perpendicularly to a base. As a slang feature, it indicates total honesty
In mixology, a "straight up" feature means a drink has been chilled (shaken or stirred with ice) but is strained and served without ice in a stemmed glass. As a slang feature