When estate agents and the like refer to the facing of a garden, they mean the direction you would look towards if you were looking through the back window to the back garden (and beyond). Generally in the UK, being in the Northern Hemisphere, South-facing is preferred for maximum sunlight throughout the whole day. The overall amount of light received depends on the aspect (the direction your garden faces). Use a compass and look out of the back window.
North-facing gardens get the least light and can be damp
South-facing gardens get the most light
East-facing gardens get morning light
West-facing gardens get afternoon and evening light