Activity Bar

The function is very similar to the panel bar in Photoshop in that it allows the user to move the cursor over the icon associated with the panel they want to open.

Let you open: Shapes, Brushes, Panoramas, Brush Parameters, Strips, Stencils, Smart Materials, Sculpt models, 3D Splines, Joints and Presets.
Lock at Windows Menu > Panels to open it separately.

You can change the position from horizontal to vertical on Edit > Preferences > Theme > Activity bar position.