It essentially means "without a doubt", "undoubtedly", "certainly", or "for sure" in English. It expresses a high degree of certainty about something.
For example:
* "Sin lugar a dudas, él es el mejor candidato." (Without a doubt, he is the best candidate.)
* "Ella es, sin lugar a dudas, la mujer más hermosa del mundo." (She is, without a doubt, the most beautiful woman in the world.)