Burglars broke into the Democratic Party offices to find out what democrats were doing. Nixon first denied he had anything to do with it.
"... in all of my years in public life I have never obstructed justice...People have got to know whether or not their President is a crook. Well, I'm not a crook." 
It looks like he was a crook. He certainly tried to hush the burglary up whether he knew about it before or not.
"We could get that. On the money, if you need the money you could get that. You could get a million dollars. You could get it in cash. I know where it could be gotten. It is not easy, but it could be done. But the question is, Who would handle it? Any ideas on that?" -- Nixon to John Dean, March 21, 1973. 
For that he was charged with obstruction of justice. Did Nixon actually authorize the break-in? We’ll probably never actually know for sure. One sensitive tape that might have held that information was erased. Nixon’s friends insisted that was an accident.