Tajweed errors are broadly divided into lahn jali (obvious error) and lahn khafi (subtle error). This is not merely a distinction between more and less melodious recitation.
An obvious error affects the basic form of a letter or word, such as its identity or vowel.
A subtle error affects a finer recitational quality or manner of articulation while the basic letter remains recognizable.
The distinction helps direct correction: is the word itself being altered, or is an otherwise correctly identified letter missing a required quality?