A tongue twister is a phrase, sente

A tongue twister is a phrase, sentence or rhyme that presents difficulties when spoken because it contains similar sounds - Whistle for the thistle sifter, for example. To get the full effect of a tongue twister you should try to repeat it several times, as quickly as possible, without stumbling or mispronouncing.
Tongue twisters have long been a popular form of wordplay, particularly for schoolchildren, but they also have a more serious side - being used in elocution teaching and in the treatment of some speech defects.