The primary colours are Red (R), Blue (B) and Green (G). They are at the corners of the triangle. If you mix the 3 primary colours, you get White (W). White is at the centre of the triangle.
R + B + G 
W
Secondary colours are obtained by mixing 2 primary colours together. The secondary colours are on the sides of the triangle between the 2 primary colours from which they are formed. They are Magenta (M), Yellow (Y) and Cyan (C).
R + B
M
R + G Y
B + G C
Complementary pairs are a primary and secondary colour mixed to give white. Using the triangle, mix the secondary opposite the primary to get white. For example Blue and Yellow are complementary pairs. If you think about it, this makes sense as Yellow is made up of Red and Green. So mixing Yellow with the third primary, Blue, gives White.
B + Y
W
R + C W
G + M W