Addressables are a new feature by Unity3D to allow the loading of an asset by a simple address no matter whether it is stored locally or remotely. Required assets are loaded at runtime which can reduce build time, installation size and Unity Editor play times. It can also be used to deliver content for deployed games and applications.
However, the setup and the content update after you deployed your game / application can be challenging.
The manual is found here: https://docs.unity3d.com/Packages/com.unity.addressables@0.6/manual/index.html