It's ok to do something to one side of an equation as long as the same thing is done to the other side. The two sides will remain equal. Anything divided by itself is equal to 1. A very common circuit is a voltage divider. The total resistance is simply the sum of the two.
Capacitors are devices which have metal plates separated by an insulator.
They are used to temporarily store an electrical charge. Microfarad means millionths of a Farad. It's abbreviated mf, MF or some variation. A value without a designator is assumed to be in microfarads. For example, in the schematic you will see several capacitors simply designated .1.