This attribute asks the PHPTAL parser to ignore the elements's open and close tag, its content will still be evaluated.

<span tal:omit-tag="condition">
  if the condition is true, then only this text will appear and span open and close will be removed

Will produce:

only this text will appear, span open and close will be removed

This attribute is useful when you want to create element optionally, e.g. hide a link if certain condition is met.

If you want element that is never output, you can use tal:block

<tal:block tal:repeat="x php:range(1,10)">only this text will appear, ten times.</tal:block>