His explanation was crude, and I didn't really understand it.
粗い (あらい) is a different adjective meaning 'coarse' in texture or spacing, but 粗 (そ) is a bound morpheme used in compounds and as a na-adjective.
crude; rough; shabby; poor-quality
coarse; crude; inferior
poor; lame; ill-prepared
rough; wild; rude; violent
carelessness; blunder; careless mistake · wetting oneself; soiling oneself
Standard kanji spelling.
Outdated kanji variant; rarely used in modern Japanese.
Outdated kanji variant; rarely used in modern Japanese.
The kanji 粗 originally depicts unpolished rice, symbolizing coarseness or lack of refinement. The reading そ is the on'yomi, used in many compounds.
osteoporosis
crude manufacturing; shoddy production
bulky waste; oversized garbage; large refuse · useless husband; retired nuisance husband
rough; crude; unrefined
simple diet; plain food; frugal meal
coarse; crude; rough
small gift; token of appreciation · inferior goods; low-quality article
density; coarseness and fineness; sparseness and denseness
coarse cloth; rough fabric
careless; thoughtless; rash; rude
rough sketch; rough drawing
rough; careless; slovenly
plain clothing; coarse clothes
brushwood; twig
raw oil; unrefined vegetable oil
crude; coarse; rough