In Texas Hold 'Em you are dealt 2 cards, you bet, then 3 community cards are dealt(the flop), you bet, a 4th community card is dealt (the turn), you bet, a last community card (the river) is dealt, and the final round of betting occurs.
To catch on the flop is when your hand improves comparatively after the dealing of the flop. One of your cards pairs up, your connectors become a straight, your suited cards become a flush, or your pocket pair improves to a set.
Extending the analogy, to catch on the flop is take a reasonable risk, which pays off. To catch on the river, the last card, would be to take a less reasonable risk, which pays off. If you are playing a speculative hand one that needs to catch to have a reasonable chance of winning, it is usually wisest to fold if you do not catch on the flop. You have only 2 more chances to catch, the turn, and the river. If you hope to win by bluffing, then it may still pay to continue playing if you do not catch on the flop, but in that case you would have to raise, not merely call.
If you do bluff, or simply decide to stay in on a speculative hand you may still catch, if not on the turn then on the river, after all,
The river gives, and the river takes away. But that is not likely a safe bet.
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