After you enable page editor, you will find a special menu item "Adavanced" in the dropdown menu of any block container. After you click it, you will see a warning sign "Note: please stop if you don't know HTML! Do not touch anything inside %% or you will get your site screwed up." As the general rule, if you are not familar with the raw HTML editing then please don't use this feature. Also you don't want to touch anything wrap around %%.
Here is a brief introduction of what's wrapped inside %%.
1) a digital numbers e.g. %%494%%
This is block id. It means loading the content of block Id 494 in this place.
2) PlaceHolder and digital number e.g. %%PlaceHolder1%%
This is where you see the plus icon where you click to add a new block
Now let's talk about how you could do some interesting work to archive another layer of customization. You could add or update the HTML content outside %% tokens. Here is an example of changing "Top Master Block" in the default theme to three columns style.
If you click the "Advanced" menu, you will see the following content.
<div class="uppermenu">%%493%%</div>
<table cellspacing="0" cellpadding="0" style="width:100%;">
<tr>
<td style="vertical-align: bottom;%%width,Left Column Width,,%%">%%494%%</td>
<td style="padding-bottom: 20px; vertical-align: bottom;%%width,Right Column Width,90%,%%">%%PlaceHolder1%%</td>
</tr>
<tr><td style="padding-left: 50px" colspan="2">%%211%%</td></tr>
</table>
%%495%%%%PlaceHolder2%%
This is the block looks like.
You can change the content to following.
<table cellspacing="0" cellpadding="0" style="width:100%;">
<tr>
<td style="vertical-align: bottom;%%width,Left Column Width,,%%">%%494%%</td>
<td style="padding-bottom: 20px; vertical-align: bottom;%%width,Center Column Width,40%,%%">%%PlaceHolder1%%</td>
<td style="padding-bottom: 20px; vertical-align: bottom;%%width,Right Column Width,30%,%%"><div class="uppermenu">%%493%%</div></td>
</tr>
<tr><td style="padding-left: 50px" colspan="2">%%211%%</td></tr>
</table>
%%495%%%%PlaceHolder2%%
Then you will see the Login Menu still floating on the top rather than in the third column. This is because css class uppermenu is using absolute position.
Now go to Site > Templates -> Default.css and add the following to override the default uppermenu style sheet.
div.uppermenu {
position: relative;
}
Then press "Ctrl -F5" in the your favorite browser to force reload css style sheet.