In essence, a phatic expression is any communication whose pragmatic meaning far overshadows its semantic meaning. More than anything, phatic expressions are used to open up channels of communication and social interaction, rarely to actually communicate the literal meaning of what is being spoken. In other words, its primary function is not to actually communicate information but to communicate a feeling.
The structure of language is amazingly full of nuance. When what you're saying isn't as important as how you're saying it, that's what linguist called a phatic expression.