The toolbar buttons may be active or inactive.

Active buttons allow you to perform an operation at the current location / on the current item. An active button changes its border and background colors, a tooltip will be displayed when the mouse pointer is hovering over the button.

A button is usually inactive if its operation is not allowed at the moment (for the current element). Inactive buttons have a greyed out background and do not respond when clicked. For example, the list buttons are inactive when you work with headings. The reason for this is that a heading can never contain a list item.

Online editor: active and inactive buttons.

The screenshot above shows a heading which is being edited (the cursor is placed inside the word "Documentation", which is a heading). Note that the buttons used for creating lists are inactive because these operations can not be used with headings. The "Bold" and "Italic" buttons are also inactive because bold and italic formatting is not allowed for headings.