Tap on the Brush icon to start drawing. Tap again to open the Brush panel and have access to additional brushes and their options. 
Tap on the Eraser icon to erase. Tap again to open the Eraser panel and have access to additional eraser brushes and their options. 
The paint bucket icon will allow you to fill an area with pixels. 
The transformation tool displays a transformation box and more options to transform the pixels. 
Tap on the selection icon to select an area to transform or to isolate an area to draw on. Once an area is selected, tap on the icon again to deselect. 
The undo icon cancels the last action. Hold it to quickly undo multiple actions. 
The redo icon restores the cancelled action. Hold it to quickly redo multiple actions. You can change the number of undos you can do in the General settings, change the value next to Maximum number of undos.
An additional side bar is displayed with the options of the selected tool.
When you are using the brush or eraser tool this is where you adjust the size and opacity of the brush.
When you are using the selection or transformation tool, you have access to different options of selection and transformation.
Play and Pause the animation. 
Tap on this icon to add a new drawing sheet after the current drawing sheet of the current layer. 
Sends you to the previous frame in the timeline. 
Sends you to the next frame in the timeline. 
Sends you to the first frame in the timeline. 
Sends you to the last frame (of all your existing sheets and/or clips) in the timeline. You can choose to have the side bar on the right or on the left of the screen, to do so, in the General settings, switch the Flip side bar position button.
