In the example of mobile phones, portability means that a client can keep his or her phone number when he or she changes providers. In software, portability means whether or not a software can continue working on a new operating system. Data portability also refers to whether or not data can be transferred.
Portability is the ability of a given element (product, service, IT tool) to adapt to a new environment such as a new device or operating system (OS). It is a fundamental principle of GDPR.