Satanic messages are supposedly signs or imagery that supposedly has a pro-Satanic message. It is often imagined to appear in widely available media, include:
- Energy drink branding 
- Product bar codes 
- Satanic imagery and alleged "backmasking" (audible if played backward) in music.
Most of these alleged messages are an error in human cognition called patternicity, in which random patterns are mistakenly interpreted as meaningful.
There is no coherent motive for such messages. Satan does not exist and cannot be summoned by messages or any other means. If Satan (or God) wanted to send people a message, they probably would be more unambiguous about it.
Satanic imagery and occasional backmasking used in the music industry is a harmless aesthetic choice, and is more about challenging taboos than having any spiritual significance. Most instances of backmasking do not have Satanic messages.
Occasionally, Satanic messages actually have been deliberately added to a product, such as caramel drizzle on coffee.  These are often the result of vandalism rather than a global conspiracy.