What does it mean to be part of an "oppressed", "dominate" and "subordinate" group?
Well, dominant (the adjective, not "dominate" = a verb) means you are in charge (as a group); subordinate means you're anything but the dominant group, and oppressed means you are at the bottom -- you get the worst treatment.
Like, if you're oppressed, they all would even ignore your questions!
It's noteworthy that people may move from one of these groups to another, although they tend to stay put in one group or another as adults because of their personalities and social circumstances.
People in charge (dominant) tend to develop quirky or obnoxious characteristics "Power corrupts, absolute power corrupts absolutely"; an important part of your education is learning about this tendency and developing character (the ability to balance what is right for individuals, including yourself, with what is right for groups -- the biggest group of which you are a part is the entire ecosystem of Earth, you should note!, regardless of whether you are oppressed or dominant).