“I predict that she'll win by a landslide.”
You're talking with a friend about an upcoming election. You think that one of the candidates is a lot more popular than the others. You say this.
I predict that she'll win by a landslide.
"Predicting" something means saying what's going to happen, before it happens. For example, meteorologists try to predict the weather, and stock brokers try to predict the prices of stocks.
"I predict that ___" is a way to formally announce what you think is going to happen in the future. For example:
I predict that 2012 is going to be our best year ever!
I predict that he won't even show up to the first meeting.
When you start a prediction with this phrase, you seem confident.
In an election or other voting situation, "winning by a landslide" means winning by getting a lot more votes than the other sides. The number of votes that constitute a "landslide" depends on the situation.
A "landslide" is what happens when rocks or dirt roll down the side of a mountain and cover things up. The phrase "win by a landslide" comes from this idea. You can imagine someone getting so many votes that the votes cover them up.
The opposite of "winning by a landslide" is "just barely winning" or "winning by a tiny margin".