Basically you plant in a raised garden bed that you can reach every plant without stepping on so that the soil is not compacted. My beds are 1.2m by 2.4m.
Divide the garden bed into sections 30cm by 30cm, hence the square foot. In each bed I get 8 rows of 4 squares giving a total of 32 squares
Plant seedlings or seeds using the plant spacing number and ignoring the row spacing. For example, Onions have a plant spacing of 10 cm - so leaving 10cm between each plant you can get 3 rows of 3 plants in a 30cm square, which gives you 9 plants.
The idea is that you get more plants in a smaller space, and less weeds as there is less open space between plants, less work as a smaller total garden space, more variety, bigger yield. You can also save water as it is a smaller area to cover – I use a large watering can, and usually fill it twice.
You need to top up the beds with organic matter on a regular basis as the intensive planting takes a lot of nutrients out of the soil.