An amount of data equal to approximately 1 billion bytes.
Data size is also often measured in MB:
1 GB = 1024 MB
As an amount of data, GB is often used to refer to how much data (how many photos, videos, apps, etc.) can be stored on a device or storage medium. It can also refer to how much data is transferred over a connection, such as how much data might be downloaded from the Internet to your phone in a given month.
GB is not to be confused with Gb, which stands for gigabits. Gbps and Mbps refer to speed of data, while GB and MB refer to amount of data.