In the Editor some formatting settings can only be applied to block-level elements. From the point of view of document structure, block-level elements are elements that may contain other elements, both block and inline in type. They typically begin a new line and, when some types of (block-level) styling are applied to one element within them (one letter, one word), this styling is automatically taken over by the whole block. Some examples of block elements are a paragraph, a heading, a table, a list, or a block quote.
Inline elements, on the other hand, are minor building blocks that are contained within block-level elements and are distinguished from the text that they are embedded in mainly due to stylistic distinctiveness such as italics, bold, or fixed-width font.
Since these distinctions might be confusing for the user who is not well versed with HTML code and its structure, and as a result some users may not be sure why a given formatting feature works on a whole paragraph and another just on selected text, the Editor includes a Show Blocks feature.
When you press the toolbar button, the Editor will display a grid of dotted frames around the block-level elements of the document along with their HTML tag, like p for paragraph or h3 for level 3 heading, as shown in the figure below.