In BaZi (八字, the Four Pillars of Destiny), your birth is written as eight characters — a Heavenly Stem and an Earthly Branch for the year, month, day, and hour. The Heavenly Stem of your day pillar is your Day Master (日主 / 日元): the element that stands for you. Everything else in the chart is read in relation to it.
There are ten possible Day Masters — the yin and yang forms of Wood (甲乙), Fire (丙丁), Earth (戊己), Metal (庚辛), and Water (壬癸). A 甲 Wood Day Master reads like a tall tree — upright, growth-seeking; 丁 Fire reads like candlelight — warm, precise, personal. None is 'good' or 'bad'; each is a starting nature.
A Day Master is called 'strong' (旺) or 'weak' (弱) depending on how much the rest of the chart supports it. This is not a score — it tells you which elements bring you into balance (your 用神, favourable element), which is the practical heart of the reading.
Your chart doesn't sit still. Every ten years a new 大运 (luck pillar) shifts the elemental weather around your Day Master — which is why timing matters as much as the natal pattern.