Deep Throat was Mark Felt and he was the whistle blower in the Watergate scandal. He was the person who talked to Bob Woodward in the parking ramp at night when nobody was around. He knew because he was the Associate director of the FBI. They called him Deep Throat in order to keep it anonymous.
Woodward and Bernstein started reporting a story about a break in to the democratic headquarters and when they dug deeper into the story they found out more people were involved. They went to houses, offices, made calls and visited people in private places in order to get what information they had to offer. Some of their articles would get published along the way but nothing was as big as their last article which had the whole story to it because of Mark Felt. Some of the higher up people got mad when they were stuck or didn't write something the right way but they still kept trying to get more sources to make their article credible and sufficient. After they printed some smaller articles which started to make people wonder how well they can trust the government, they printed a big article that was the front page which had the whole story to the Watergate scandal in it. This sparked outrage with the republicans who lied but later on Woodward and Bernstein were right and Nixon and some other people involved in Watergate resigned. If it wasn't for their reporting we may not even know anything about Watergate and other presidents may have cheated as well because people would have been oblivious to it just as they were during Watergate.