How well a solution to some problem will work when the size of the problem increases.
For example, a central server of some kind with ten clients may perform adequately but with a thousand clients it might fail to meet response time requirements. In this case, the average response time probably scales linearly with the number of clients, we say it has a complexity of O(N) ("order N") but there are problems with other complexities. E.g. if we want N nodes in a network to be able to communicate with each other, we could connect each one to a central exchange, requiring O(N) wires or we could provide a direct connection between each pair, requiring O(N^2) wires (the exact number or formula is not usually so important as the highest power of N involved).
On a database, my first ideas are: - number of rows in a table - number of tables in the database - number of simultanous users
An other question might be, whether a change in the hardware (bigger/ faster drives, faster network hardware, more and faster RAM, more and faster CPU(s) clustering, ...) will lead to nearly equivalent effects in the usability of the DB - faster responses, more data/ users might be managed accordingly, ...