OAuth is a standard protocol that allows access to an API on behalf of the end user, without giving away their information, such as password, API key ID.

Each app (OAuth client) gets a token with access scope—end users will also be able to see what information the app is accessing for transparency and security.

What is OAuth Support? An Easy Way to Offer Commission-Free Trading Directly on Your App!
Add Alpaca’s commission-free trading capability inside of your app with the minimum effort through a commonly used integration method of OAuth.
OAuth Integration Guide - Documentation | Alpaca
Alpaca supports OAuth for anyone who builds apps and services using trading API. Alpaca API lets you build and trade with real-time market data for free.