Consonant Clusters, 겹받침, are two consonants that appear in succession. Don’t be confused with Double Consonants, 쌍자음, that we covered previously.
Consonant Clusters are not commonly used in Korean words, but they do sometimes appear in writing in *final consonant position. In such cases, one of the sounds must always drop (or otherwise move to another syllable. See here.) See examples below.
*final consonant : 받침