to drill, to exercise; to practice, to train
Phonosemantic compound. 糹 represents the meaning and 柬 represents the sound. Based on the original meaning "boil and scour raw silk" to make it softer and whiter. The meaning later shifted to "to make perfect" and "to practice".
《說文》：“練，湅繒也。从糸，柬聲。”
boil and scour raw silk