The toolbar is made of multiples items that simplify scene's manipulations.
Here the possibilities offered by each group of tools:
The Rendering tools
When final rendering is enabled, start/stop will open/close the render editor.
The Create tools
Add new elements to the scene.
Switch between selection gizmos.
The Wireframe tool
Display the scene in wireframe mode.
The View tools
Display the scene from different angles.
The Locking tools
Retain/restore the camera properties.
Very useful when you have found a good spot.
The Focus tool
Focus on the current selected meshes.