Area measurement you make by multiplying two linear length measurements by each other to get a surface measurement. A surface measurement is a two-dimensional measurement that you can use to measure the size of a room, a wall, a building lot, land parcels, water bodies, and anything else you need to know the surface area of.
A box 5 feet wide and 6 feet long = 30 feet. The result you write as 30 square feet or 30². There are also specific names for land area measurements, acres used in the US and Imperial systems and "hectares" in the Metric system.