Changing the subject of a formula on the mathisfun.com page refers to rearranging an equation to solve for a different variable. This can only be done using valid algebraic operations, always remembering that both sides of the equation have to be changed in equivalent ways so that the equal sign remains true.
Here is an example:
In physics we have a basic equation stating that the force carried by an object is the product of its mass and its acceleration: F=MA
This formula states that if we know the mass of an object, for example by weighing it, and we know its acceleration, by measuring the change in speed over a measured time, we can calculate the force that it took to cause that acceleration by multiplying the mass by the acceleration.
However, what happens if we know the force and the acceleration, and we want to find the mass?
Then we have to rearrange the equation, or formula, to "solve for the mass".
The original formula is F=MA, with F alone on the left hand side.
How could we rearrange it to get M alone on the left hand side?
Let's divide both sides by A, so we get F/A=M.
Now if we just switch the order, we have M=F/A, where M is the new subject of the formula.