蔽 is largely literary/classical now and rarely stands alone in modern speech. You will see it in compounds like 遮蔽 (to shade/screen), 隐蔽 (hidden/concealed), and the set phrase 一言以蔽之 (sum up in a word) from Confucius's Analects.
memory
Picture a tattered (敝) grass mat (艹) tossed over something to hide it - the radical 艹 plus phonetic 敝 gives both the form and the idea of "covering up."
艹 is the top-position form of 草. It supplies the screening sense - covering or hiding behind foliage - and links 蔽 to other plant-cover chars like 荫 (shade) and 蒙 (cover).
敝 supplies the sound bì cleanly. It also carries a faint semantic flavour - torn cloth being beaten - which combines with the grass radical to suggest hiding behind tattered screens, reinforcing the conceal-and-obstruct meaning.
