Resistance is an important concept in electrical theory. Simply put, resistance is the opposition to electric current. Insulators have a high resistance, conductors have a low resistance. Whenever a current flows through something with resistance, it loses energy (resulting in a voltage drop).
These slides outline the concept of electrical resistance, Ohm’s Law, resistors in series, resistors in parallel, and factors which effect resistance (type of material, length, cross sectional area and temperature).