What do tintable paint mean?

It means, you can add pigments to it to change the hue a bit. Of course you can do this with any paint, as long as the pigment paste has the right kind of binding fluid (waterbased paint can only be tinted with waterbased pigment or coloring fluid). It can also mean, that there's a specific product (pigment paste) available that' suitable to add to this kind of paint.