According to Ohm's law, V=R・I, the values of any two of the voltage, current, and resistance of resistor are known, it is possible to determine the value of the remaining parameter.
Not only lamps but also heaters, motors require energy to operate. The amount of energy required is named power consumption which is P(W)=V(V)*I(A)
 
A voltage of about 2V is applied across its two terminals can light up a LED, so one single dry cell of about 1.5V is not enough. When two dry cells are connected in series, the 3V voltage will exceed the current flowing through the LED and damage the LED.

In this case, a resistor is placed between the LED and the dry cells is necessary. It eliminates  the excessive load on the LED or battery, so electric circuits can operate smoothly by limiting the flow of current. A resistor is an essential component in any electric circuit.
 
As the power will entirely be radiated as heat, if the power consumption is high, the temperature of the resistor itself increases, finally the resistor might be burned it or melted down. So it is necessary to define the amount of power the resistor can safely consume. This power is called the rated power of the resistor. Normally, engineers choose the resistor rated at 2 more times of the power consumption.
 

In terms of serial connection, the resistance value increases as the number of resistors in the connection. While in parallel connection, there resistance value decrease as the number increase of resistors.