Market timing
Trying to buy low and sell high by prediction.
Market timing is trying to jump in before prices rise and get out before they fall. It sounds sensible and is very hard to do well.
Why it rarely works
You have to be right twice: when to get out and when to get back in. Many of the market's best days have come right after its worst ones, often when timers were still waiting on the sidelines.
Dollar-cost averaging is the opposite approach. It accepts that nobody knows what next month brings and simply keeps buying.