It emphasizes a condition that makes something even more likely or important.
Example:
* "J'ai besoin de vacances, surtout si je dois continuer à travailler aussi dur." (I need a vacation, especially if I have to keep working this hard.)
You can also use "surtout" by itself, meaning "above all" or "most importantly".
Example:
* "Surtout, n'oublie pas de prendre ton parapluie." (Above all, don't forget to take your umbrella.)