A tongue-twister is a phrase that is designed to be difficult to speak properly.
Usually tongue-twisters rely on similar but distinct phonemes (e.g. s and sh), unfamiliar constructs, or other features of a language.
Tongue Twisters, amazing fun with friends and family
Tongue Twister of the Day
I see a sea down by the seashore. But which sea do you see down by the seashore?