When something's scalable, it's possible to change its size to suit your needs. Your lemonade stand is scalable if you can easily, quickly make more lemonade and obtain more cups to meet increased demand.
You can use this adjective in two ways. First, it can mean "adaptable" or "able to grow." This is often used in business, but you can describe many things this way. Your bird feeder is scalable if it works for a single sparrow but can also fit a dozen grackles. Things are also scalable when you can climb over, or scale, them: "The hill on the obstacle course is low and easily scalable."