The Americans use this phrase a lot.
but I am not sure how to use it .
A: Are you tired or something?
B: Not really.
Here "Not really" means "not at all" or " I am just a little tired."
It more or less means 'nothing out of the way'.
You might be a bit tired but no more than usual.
At least it's not a matter of concern or complaint.
Re: Not really.
To me, not really means something like, Truthfully? No or Technically? Yes. But....
Originally Posted by zhengl_2000
A: Are you tired?
B: Not really. (i.e. Truthfully? No. I'm not really tired. I'm just bored.)
A: Do you love me?
B: Not really. (i.e. Truthfully? No. I do not really love you.)
A: Are you married?
B: Not really. (i.e. Technically? Yes. But I'm not really married; I'm separated.)