Growth is a statistic your Town can have and it determines how fast your Farms can grow and how fast your Units can train.
You can receive Growth for every culture chunk in the town. This means that if your towns expand and grows a Cultural Level then you will receive more growth from those chunks.
The best biomes for Growth are the River and Plains biomes. You can view the growth rates from all biomes on the Culture page.
You can scout the stats of your town by typing the command '/t survey' in-game. This will show how much Growth, Hammers, Happiness and Beakers your town will have when you settle that town in that culture zone. Consider 75 to 125 a good amount of growth when you settle.
Some structures will increase your Growth by a set amount. For example, the Lighthouse will increase the base Growth of your town by 200.
A good way to get more Growth in your town is getting a higher Happiness level. Growth is affected by Happiness and will get a good buff if you get more happiness as well.
You can also research technologies like Exportation which increases the Growth Rate by 10%. There are some policies as well that will increase your growth (rate).