tal:replaceThis attribute will replace the entire tag with a value, or by nothing if no value is given.
<span tal:replace="string:this beautiful string">
this ugly string and span
</span>
Will produce:
this beautiful string
tal:replace can also be used to create samples in source templates, but remove them from final output.
<table>
<tr tal:repeat="item myresult">
<td tal:content="item">item value</td>
</tr>
<tr tal:replace="">
<td>sample 1</td>
</tr>
<tr tal:replace="">
<td>sample 2</td>
</tr>
</table>