Dry board refers to community cards that are not connected to each other, meaning there are very few potential draws.
A great example of a dry board would be Kh 7d 2s. In this case, the board has no flush or straight draws. Moreover, it is even unlikely that players could have many two pair combinations since community cards are far apart from each other, meaning players usually fold hands like K2 or even K7 and will not have them in their ranges.
Therefore, it is very hard to represent a strong hand on this board when you are just calling preflop so the preflop aggressor can very effectively continuation bet with most of his hands and turn an instant profit. It is worth mentioning that you should usually use small bet sizing on such boards.