In an equation, if a variable (symbol) is expressed in terms of others, the variable is called the subject of the equation. For instance, in the equation y = 4x - 8, y is the subject of the equation. Similarly, in the formula (equation)

I is the subject of the formula.

Note that

can be rearranged to express V in terms of I and R. That is, V = IR for which V is the subject. Similarly, y = 2x - 6 can be rearranged to an equivalent equation

for which x is the subject.

Given an equation, when a variable other than the current subject is required to be a subject, then we have to rearrange the equation to get the new subject. The task of making a new subject by rearranging a given equation is called change of subject.
In other words, making a given variable the subject of an equation means expressing that variable in terms of the others.