烁 is a classical and literary character rarely used alone in modern Chinese. It almost always appears in the compound 闪烁, meaning 'to flicker' or 'to twinkle'. Encountering 烁 solo is a sign of classical or poetic text.
火 (the fire radical) supplies the sparkle and glitter - 烁 is what flames or stars do, flickering and shining. It groups with 闪 (flash), 灿 (brilliant) and 煜 (glow) in the family of light-emission chars.
乐 has the alternate reading yuè (music), and 烁 takes shuò by shifting the same phonetic stem found in 铄 (shuò, melt metal). The yuè/lè/shuò triple reading of 乐 is one of the more memorable sound-shift cases in the language.
