Need Help Defining Symbols

I’m doing some tutorials and after doing multiple searches online and on the forums, I can’t find any answers to this. What do the symbols -= and += mean?

public void MoveGem(Gem gemToMove,Vector3 toPos, Vector3 fromPos)
{
Vector3 center = (fromPos - toPose) * .5f;
center -= new Vector3(0,0,.1f);
Vector 3 riseRelCenter = fromPos - center;
Vector setRelCenter = toPos - center;
float fracComplete = (Time.time - startTime)/SwapRate;
gemToMove.transform.position = Vector3.Slerp(riseRelCetner,setRelCenter,fracComplete);
gemToMove.transform.position += center;
}

They are simply a short-hand way to add or subtract a value from some variable.

-= subtracts the value on the right from the variable on left and stores the result back in the variable.

+= does the same, but adds the value. So…

int a = 10;
a += 1;

The above code adds 1 to the value in a. When it completes, the value of “a” will be 11.