The element and its content will be shown only if the condition is evaluated to true.

<p tal:condition="identified"> Welcome member … </p>
<p tal:condition="not: identified">
  Please login before accessing this page

If the PHP backend does not provide your templates with enough methods, you will often have to fall back to PHP for special conditions:

<span tal:comment="show only if more than five items in the cart"
      tal:condition="php: cart.countItems() GT 5"></span>

This may put too much logic inside the template and it is sometimes preferable to provide boolean attributes or accessible methods to the template:

<span tal:condition="cart/hasEnoughItems"></span>