A consonant cluster in a word is a group of consonants with no vowels between them. The longest possible cluster in English is three consonant sounds at the start, such as 'splash', and four at the end, as in 'twelfths'.
The tongue twister 'The sixth twisty crisp' has several consonant clusters in it, making it difficult to pronounce.
In the classroom
Consonant clusters cause problems for learners whose first language does not allow so many consonants together without intervening vowel sounds. Examples of this are Spanish and Arabic.