Palm sugar is a sap from palm tree flowers like date, coconut or sugar palm. It is used in many cuisines in tropical countries, like Thailand, Indonesia, India.
It is a bit less sweet than a brown sugar and it has caramel flavour. If you do not have it, substitute with brown sugar. If you decide to substitute it with white sugar, not only, that you have to use much less white sugar, as it is much sweeter than a palm sugar, but the dish will miss the specific caramel flavour.