"you are so mean" it means that you are rude, not behaving properly, ex. cursing. "that's do mean" its incorrect, however it's correct form would be "that's too mean", meaning you are very rude, simillar to the first one but it is used when someone crosses the line, for example cursing and hitting you.

"you are so mean" = you are unkind "that's do mean" is grammatically incorrect. It makes no sense. Maybe you learned this from a non-native English speaker.

Does it mean that one person who does't want to help or give something to others?