Projects in Sampler act like packages that can store multiple assets. The Project panel displays the assets that make up your current project.
Click one of your assets to load them in the Viewport, or drag an asset onto the Layers panel to use it in other assets.
Use the + next to Materials to add new a new material to your project. Similarly, use the + next to Environment lights to add a new environment light to your project.
Right click an asset to rename, delete, or duplicate it. You can see some metadata about the asset in the right click menu.