A slope of 2:1 means the hillside drops 1 foot for every 2 horizontal feet. It describes how steep the hillside is. A slope of 2:1 is considered to be a very steep hill. Many plants have trouble surviving on hills this steep because the water runs off.
A slope of 3:1, meaning the hillside drops 1 foot for every 3 horizontal feet, is a better slope for landscaping. Lawns can be mowed on a hill this steep, and drought-tolerant plants can be cultivated. Less steep slopes are easier to landscape. Irrigation on slopes needs a slow-drip system to prevent the water from just running off down the hill.