A minor in possession is anyone under the age of 21 who is in possession of alcohol. Many states have laws regarding what happens if a minor has alcohol; these laws are meant to help educate minors on how dangerous drinking and driving is, to help involve minors in community services, and to help those who need chemical dependency treatment get that help.
MIP violations can lead to a number of penalties depending on the case. For instance, you may lose your license for up to a year in some states, like California. If you don't have a license, you may not be able to apply for one until several months or a year following your conviction. In most cases, there are a variety of penalties you could face, so working with your attorney to defend yourself can help you reduce or eliminate penalties from your sentence.
You don't necessarily have to be drunk to be found guilty of being in possession. If you're under the age of 21 and caught with alcohol, you can be arrested. If you drank alcohol, attempted to buy it, or if you had it in your possession, then that's enough to convict you in many cases.
Fortunately, there are defenses to these charges. For instance, in some states, you can drink with your guardians when you're under age, so a test for alcohol on your breath wouldn't indicate anything negative necessarily. If you drink at a religious ceremony, that is often a good reason and a possible defense. If there was no alcohol in the container found, that can be a great defense for you, too.
Source: FindLaw, "MIP: A Minor in Possession," accessed March 30, 2016