It means to bring something, typically liquid or steam, to a temperature high enough to cause injury to someone or to change the constitution of the liquid. It may be short of boiling. So you talk about being scalded (rather than being burnt, which suggests a dry heat) by boiling water or steam, or you talk about scalding milk if you are making a custard or something like that.

Hello Quyen, Used as an adjective it means extremely hot. Like tea that just came off the stove is scalding hot. Scald can also be used as a verb. You scald fruit or tomatoes by placing them in water that has almost reached a boiling temperature so you can remove the skin more easily. I hope this helps.