Data portability is a concept of making it easier for users to move their data from one online service to another. It's part of the broader concept of interoperability, which refers to how different applications can work together.
Data portability is important because it allows you to control your data. You should be able to get your data out of an application and use it somewhere else, if you choose.
Why Data Portability Matters
The ability to take your data from one application and use it in another can be useful for several reasons:
If you're unsatisfied with one service, you may want to try something new. The easiest way to do this is by exporting your data from one service and importing it into another (after signing up for the second service).
If you're using multiple services at once, having access to all your data in a single location makes managing it much easier. For example, imagine if all your photos were stored on Facebook, but some of them were also stored on Flickr — it would be difficult to manage if all these services had different interfaces and kept separate copies of your photos.