When spoken to children, this phrase communicates a gentle reminder to speak clearly and enunciate. When spoken to adults, this phrase communicates both insult and an urgent desire to be kicked or punched squarely in the nose without any hesitation.
For this reason, careful consideration should always be made about the approximate age of the person speaking, not only out of respect but in avoidance of severe pain, injury, and bloodshed assuming none is wanted.
Please speak more clearly. I'm confident you can do this. Just use your words.