They don't have slightly different meanings, they have completely different meanings.
"Du" is singular you
"Ni" is plural you
Some people might also tell you that "ni" could be used for formal singular you, but this not entirely true. I don't think it's very controversial in Finnish Swedish, but in Sweden many people, especially older people, might get offended if you try to use it as a formal you. Traditionally it was used as a marker for status and inequality, and this is something modern Swedes are trying to move away from. To show that everybody is equal, we prefer to use "du" in all cases where the word "you" is singular. Therefore I recommend to refrain from using "ni" as a formal singular you.
what is the point of accent or Umlaut above the "ater" ?? "ä" is it necessary?
If you're on a computer, you should see a row of circles underneath the text box. You can click on these to get the letters. Otherwise, you can hold down the Alt key and type in the number codes for each letter. They are: ä = alt 132 å = alt 134 ö = alt 148 á = alt 160
Hope that helps!