Correlated events are events where the relationship between one outcome and another is very strong...if A happens then B is likely to happen as well.
Using a baseball game as an example, let's say you want to take the Boston Red Sox to win on the money-line and the Boston Red Sox on the run-line. If the Boston Red Sox win the game, it is also likely that they will win by at least two runs (the run-line is always +/-1.5) therefore the selections are said to be ‘correlated’ and cannot be parlayed together. For the same reason, we do not allow the favorite/underdog on the runline and the over/under of the same game to be placed in the same parlay.
Similarly, you cannot parlay the line for the first half of a football game with the line for the whole game or the total for the first half of a football game with the total for the whole game, as the two are considered ‘dependent’.
Less obviously, but nonetheless correlated, are the favorite/underdog on the spread and the over/under of the same game placed together in a parlay, teaser or IF bet when the spread is greater than or equal to 30% of the total.