What is that in layman terms?
If a handicapper is able to accurately select bets that have value, parlaying these plays will result in a higher return on investment in the long run, but also increase the risk of ruin (bankroll depletion) and variance of returns (resulting in an increased probability of being down X amount at a given point of time).
However, if a handicapper is unable to accurately spot value, and parlays negative EV bets, the risk of bankroll depletion and variance of returns will also result, but so will the increase in negative returns on investment.
In a nutshell, if you are not an advantage gambler and betting recreationally and want to avoid exhausting your bankroll, do not parlay bets.
If you are a proven advantage gambler, parlaying bets will increase bankroll growth optimization, but should still only have parlays be a small% of their nominal bet size.
When you see some someone claim parlays are suckers bets or for losers, these people simply don't understand the underlying mathematics behind them, and should be encourage to learn more before giving out falsified information.