Definition
In a standalone build the Data folder contains globalgamemanagers and its assets file. Unity stores serialized settings there: the build scene list, the Unity version, the company and product name, input and physics settings, and which engine modules are present. It is a regular Unity serialized file, the same format Unity uses for assets.
Relevance
This file is the project's settings inside the player. The scene list, the Unity version, and the enabled modules are stored there as serialized data, so the build order and the product name can be read from the Data folder without opening a script.
Example
The Data folder of a shipped build contains globalgamemanagers. It holds the scene list in build order: Boot, MainMenu, Shop, Level_01. That is the order the player uses to load scenes.