Language is a system of communication that combines symbols, such as words or gestural signs, in rule-based ways to create meaning. Language serves several crucial functions such as the transmission of information and the ability to express our thoughts about social interactions. It is also one of the few documented cases in which children are more efficient learners than adults. We, as humans, spend much of our conversational time establishing or maintaining our relationships with others, which, in turn, creates a bond between two people
One commonly held belief is that this special "bond" enables twins to invent their own secret language that only they can understand. This phenomenon, known as cryptophasia, is natural to expect when two people have been together from the moment of conception. However, this notion is not as truthful as it may seem. Cases of cryptophasia among twins turn out to be a result of phonological impairment and other types of language delay that are more prevalent among twins. The twins are simply attempting to use their native language, but with poor articulation and significant pronunciation errors. The reason they make it look like they have their own "language" is because twin pairs tend to make similar kinds of phonological errors, making their speech more understandable to each other that to their parents or nonrelated children.
Video of two twins talking in their "own" language.