I don't understand why Trump. Can someone explain it.?

I would say that beyond his celebrity--which has something to do with it--the two biggest reasons some people seem to legitimately support him are:

1) There is a lot of dissatisfaction with the way the country has been going in recent years, and for this reason a lot of people are receptive to the idea of electing a political outsider.

2) There are a lot of nativist voters who have been ignored by both major parties and they are happy to back a significant candidate who seems to really support their views. Actually you've seen a lot of nativism outside the US--say Marie LePen in France or UKIP in the UK--so this is just a continuation of an international trend.

And beyond that there may simply be a lot of people who just aren't paying enough attention to realize that the man is a colossal narcissist with no understanding of or interest in public policy and who will deeply disappoint anyone counting on him to enact positive reforms.

The support for Trump in the primary was mostly driven by racial animosity. There were strong correlations between Trump support and racism. For example, in the South support for Trump correlated strongly with how many slaves a county had at the time of the Civil War. Counties with more slaves supported Trump more strongly. There was also a correlation between how many times people in a state google searched racist topics and support for Trump. These people are worried about the declining racial prestige of whites and fear racial contamination from immigrants. It's not a coincidence that Trump's two biggest, and most stable, policy suggestions are a wall on the southern border to keep Mexican immigrants out and a "total and complete shutdown" of Muslim entry to the US. His supporters fear foreign contamination, whether its from Mexican immigrants or Muslim refugees. There's also a very strong correlation between support for Trump and a psychological tendency towards authoritarianism. People who score high on authoritarianism are very afraid of change and want a strong leader figure to protect them from it. Trump, with his blustery bravado, seems to fit that bill.

That explains his popularity in the primary, where Trump consistently receieved a large minority of the vote. Now that he's the nominee, many other Republicans, who did not support him in the primary, are rallying around him. There's an old saying about American presidential primaries that "Democrats fall in love, Republicans fall in line." While people opposed Trump in the primary, many of them are falling in line now that he's the nominee.

populism, nationalism, racism

same thing that was behind a lot of similar movements, for example over a hundred years ago when the so called know nothing party had a big problem with irish and catholic immigrants. the whole history of england is about one wave of immigration after another, britons, angles, saxons, normans, with frictions every step of the way. and yes hitler and his nazis were another example, a rather extreme one, and there's been lots of others. trump and his idiot followers are just a more recent instance of the same old sheeit

Because he actually follows through with his word. The latest example; He said he would pay women what they are based on skill sets. DNC supporters are using that and saying he pays women on his staff far less than men but Hillary pays women and men almost the same. They ignore the fact that on average FOR THE SAME WORK Trump pays women on his staff $1,200 more a month than Hillary pays either men or women on her staff. With Hillary paying so little and still claims she is watching out for womens interest, it gets very obvious who really is looking out for womens interest.

Obama has been "disappointing" mostly to those who don't know anything, but rather go off of right-wing sound bites and propaganda.