Trump was attempting to court more Republican voters by clinging to the "Birther" argument, a movement that believes that Obama was not born in the US. However, he's now saying that he believes that Obama was in fact born in America. Trump is therefore a liar who just wanted to get more votes.
Either he lied or he has changed his mind. And since Trump is more likely to say he never thought that Obama was born outside of the United States that he is likely to admit that he was wrong about something he lied or he his going to lie about changing his mind.
Trump lies a lot, so it's not a big jump to think that he might have lied about his beliefs to begin with. His opinions seem to evolve over time based on what he feels at the moment. Trump is like a spoiled schoolboy; very fickle. One year he's a democrat, then the next he's in the GOP. Who knows with Trump?
He said that that Obama was native to America. Trump was just recently asked at a number of standalone events with regards to his birther ideas, as well as if he believed each one of them. Additionally, he refused to disavow those ideas each and every time. Basically, he said that he does not need to talk on the subject of that any longer.