Wet boards contain many possible draws and combinations that connect strongly with the calling range. Examples include J-T-9 with two suits, 9-8-7 with two suits, or Q-J-8 with a flush draw.
These boards often require a lower c-bet frequency. The opponent has more hands that can continue, including pairs, draws, two pairs, sets, and strong straight draws.
Betting too frequently on wet boards can cause you to build large pots with weak hands while allowing your opponent to realize equity efficiently.
In many situations, checking becomes an important part of the strategy. You can protect your checking range with strong hands while keeping weaker hands in the range as well.
For players who want to understand these situations in more detail, the course section
When not to c-bet in poker common postflop mistakes provides a useful framework for identifying boards where automatic aggression becomes unprofitable.