What is UID on Android?
Is it ID of a particular user or of a particular application?
If you’re referring to this UID: Android is based on Linux, so basically it’s the same UID you have on a Unix-like OS. When installing an app / package, Android by default creates a UID specifically for that package, so that it can have its private resources / storage space. When no packages are using that UID any more (which could be shared), the UID is deleted.
This ID is particular to the application
Android assigns a unique user ID (UID) to each Android application and runs it in its own process. Android uses this UID to set up a kernel-level Application Sandbox.
Refer to this doc https://source.android.com/security/app-sandbox
UID is actually called
AID, which is used to identify the owner of a process and the owner of a resource. Binding those two together,
it becomes the backbone of Android application sandboxing mechanism.
A full and detailed description of how it works and what does it means for application need a long article, which you can find it here.