Uninterested means you are not interested in something...maybe because it's boring. Example: He was uninterested in what we had to say.
Disinterested means that you are not biased or your opinion is neutral. Example: Scientific research should be as disinterested and independent as possible in order to be trustworthy.
These words are mixed up a lot though so you will often find native speakers using disinterested with the same meaning as uninterested.