This is a clever and memorable phrase to use in a campaign about deforestation. It is simply stating that while trees can take many years to grow to their full height, it takes mere hours to cut them down and remove the good that they are doing for our environment.
Deforestation means to permanently destroy a forest. This is done either to obtain the timber to use as fuel, or to make paper, furniture or homes from. The other common reason for deforestation is to clear land, which can then be used to expand cities or build new cities.
The biggest problem with deforestation, and the reason why phrases like "years to grow; hours to go" are important is that it is one of the primary causes of climate change.