Dishes are used to receive signals beamed down from satellites in geostationary orbit above the earth. The beam covers an area of the earth known as its "footprint". Satellite dishes work by focusing the beam towards the LNB on the dish arm. The LNB then sends the information via a coax cable to your satellite receiver that inprets it and changes it into the picture and audio. Of course you have to be within the satellite's footprint to successfully receive a signal. The footprint is stronger centrally, becoming gradually weaker the further towards the edge of the footprint you get. The weaker the signal, the bigger the dish you'll need to capture enough of the signal being transmitted. You can view footprints online. Below are the footprints for the three Astra2 satellite beams - Astra2 is found at 28.2E, that's 28 degrees East of due South (around 140-145 degrees on a compass in the UK). The Astra2D carries the UK BBC/ITV programming. The Astra2 south and north beams carry, amongst other things, the UK Sky TV channels.
The elevation is also important and varies depending upon where you are. If you can imagine that the satellite at which you are trying to point the dish is stuck at a set point above the equator, the further south you go, the greater the degree of elevation required.