Resistors are used to restrict the flow of electric current. An example of their use is when a resistor is placed in series with a light emitting diode (LED) to limit the current passing through the LED.
Carbon Film Resistors may be connected either way round as they are not current sensitive, and they are not damaged by heat when soldering.
Resistance is measured in ohms, symbol ohm (omega). 1ohm is quite small so resistor values are also given in kohm and Mohm.