The Natural-born-citizen clause is one of the requirements to be president in Article Two of the Constitution. It states that in order to become president, one must have been born in the United States, or have been a citizen at the time the constitution was adopted. This clause prevents potential presidents who would serve the people from becoming president.
Some conservatives want Obama to be impeached, using this clause as an excuse, claiming that he is not a natural-born citizen. They probably just want to impeach him because he is more liberal than they are.
A few facets of this claim immediately jump out as being far-fetched: first, that a sitting U.S. Senator who has already spent a good deal of time and money securing his party's nomination for the presidency would suddenly be discovered as ineligible due to an obscure provision of U.S. law; and second, that U.S. law would essentially penalize someone who would otherwise qualify for natural-born citizenship status simply because his mother was too young. [She was 18 when Obama was born and had not resided in the USA for 5 years after reaching the age of 16] The fact is, the qualifications listed in the example quoted above are moot because they refer to someone who was born outside the United States. Since Barack Obama was born in Hawaii, they do not apply to him.As you can see these myths have been debunked.
The Fourteenth Amendment states that "all persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States." Since Hawaii is part of the United States, even if Barack Obama's parents were both non-U.S. citizens who hadn't even set foot in the country until just before he was born, he'd still qualify as a natural-born citizen.