An acre is a unit for measuring area, especially larger areas like farms. An acre is 4840 square yards.
It's pretty easy to imagine a square yard: three feet by three feet. Now imagine 4840 of them and you've got an acre. That's a pretty large chunk of land, and acres are used for measuring big areas in some English-speaking countries. Large areas such as farms, forests, and golf courses are likely to be measured in acres. Farms in particular need many acres to grow crops and give the animals space to live.