Scalability is the ability of a program to scale. For example, if you can do something on a small database (say less than 1000 records), a program that is highly scalable would work well on a small set as well as working well on a large set (say millions, or billions of records).
Like gap said, it would have a linear growth of resource requirements. Look up Big-O notation for more details about how programs can require more computation the larger the data input gets. Something parabolic like Big-O(x^2) is far less efficient with large x inputs than something linear like Big-O(x).