Richard Nixon was a Republican US president running for reelection and a group of Democrats, called the Democratic National Committee were trying to get their candidate elected. The DNC had their office in the Watergate Hotel, in Washington D.C..

One night there was a break-in at the DNC office, it wasn't a simple robbery, it was an attempt to steal documents and learn about the Democratic strategy. Lots of fingers got pointed and accusations flew quickly.

Long story short, it was in fact Nixon's administration who coordinated the break-in, the perpetrators were caught and found to be literally paid with campaign money from the Nixon campaign. Now this was bad, but the big "no-no" was the Nixon personally did all he could to cover up the crime. So it's a case less of the crime itself, more so that the President orchestrated a campaign to cover the whole thing up. The key was while Nixon was publicly defending himself and saying he knew nothing, the investigators literally got voice recordings, from the Oval Office, of the President talking about and directing the cover up process.

Many people were convicted or went to jail and facing legal action himself, Nixon stepped down from the presidency and his Vice President (who became President upon the abdication) pardoned Nixon. Ironically, had he done nothing at all, Nixon would have safely won his reelection anyway AND the act of pardoning Nixon was so unpopular it doomed his replacement, President Ford, to a largely do-nothing one term presidency.