The self
What is the Day Master?
The Day Master (日主) is the Heavenly Stem of your day pillar. It is you. Every other character is read by how it helps or spends that one self. The first question a reader asks is whether the Day Master is strong or weak, and the answer points to the elements the chart most needs to come into balance.
Strong or weak
Strength weighs three things: the season of birth (the biggest factor), whether the self is rooted in the branches, and whether it has allies among the stems.
Not good or bad
Strong is not better than weak. The aim is balance. A strong self needs outlets for its energy; a weak self needs support to carry its load. Both can be fine charts.
The useful god (用神) is the one element that best restores balance, the chart's "primary medicine." It is not a lucky charm. It is a description of what a person most needs more of, whether that is rest and learning, or responsibility, or a creative outlet, to come into balance.