Deforestation is the destruction or removal of forest, and then re-used for non-forest use. Only humans do this to clear land for homes, urban development like shopping malls, or most commonly - for farming. Wildfires that level forests are not deforestation, as there's an expectation that the forest will grow back.
Although clearing space for farms sounds like it's important for humans, it's not as simple as that. Most deforestation occurs for either large-scale commercial meat farming, causing carbon dioxide increases and climate change, or for palm oil plantations. Palm oil plantations seem like a safe alternative because they replace the naturally occurring trees with palm trees, but there are hidden consequences. For starters, palm trees do not provide the same habitat for animals that original forests provide. Second, people build palm oil plantations over existing peat bogs. Normally, peat bogs release gas through the carbon cycle on a steady basis, but palm oil cultivation can speed this process up. This also impacts climate change negatively.
Our forests are a rapidly dwindling ecosystem that are necessary for the survival of countless species of plants and animals. Of the original 6 million square miles of tropical forest, only 2.4 million square miles remain due to deforestation. If we hope to have a sustainable ecosystem for years to come, it's critical we protect our forests. Not only for the sake of the forests, but for all endangered animals and for people alike.