Many companies seeking to convince consumers of many things sell subliminal recordings—recordings meant to be played while you are asleep. This is designed to be an especially convenient method of learning—you spend exactly no time or effort at all actually trying to learn something that might otherwise be difficult and you wake up master of a new language or some other achievement.
That is your major red flag right there: it's so easy and it doesn't require any sort of change in your lifestyle. Any time anyone tries to sell you something with this kind of tagline, turn around and walk away because as the ancients used to say, there is no royal road to learning.
The Supporting Science
On top of that, there is no concrete or convincing scientific evidence for the efficacy of subliminal recordings. Keep in mind how you would have to prove that it works: One, you must be beyond doubt that your subjects are actually asleep. Two, they must be given material subliminally, as they sleep. And three, they must be able to recall it afterwards. No studies have shown that this happens.
However, this method of learning isn't entirely useless, as pointed out by Ian McLaren, Professor of Cognitive Psychology at the University of Exeter, over at The Naked Scientists.
According to McLaren, subliminal recordings may have some influence on the sleeping mind, at least enough to make it prone to suggestiveness. However, whereas McLaren is hardly enthusiastic about the efficacy of such a system but the same cannot be said of Joel Weinberger, PhD over at Psychology Today, who argues that what he terms "subliminal psychodynamic activation" is effective at boosting and reinforcing treatment modalities like psychotherapy.
There are several other purported methods for sale that make grand claims about their ability to deliver, in sum, something for nothing.
Again, red flag. There is no such thing.
The immortal Cecil Adams over at The Straight Dope addresses this issue broadly and reaches the same conclusion.