The spelling of "lit up on" can be confusing due to the different sounds that the letters can make. To break it down phonetically, "lit" is pronounced /lɪt/ with a short "i" sound, while "up" is pronounced /ʌp/ with a short "u" sound. The word "on" is pronounced /ɒn/ with a short "o" sound. When combined, the phrase is pronounced /lɪt ʌp ɒn/. It means to brighten or illuminate something, and is frequently used to describe a sudden burst of light or energy.