Option 1: Medicinal plants - This is a direct use of forests because medicinal plants are obtained directly from the forest and used for their medicinal properties.

Option 2: Checking soil erosion - This is the correct answer as it is an example of an indirect use of forests. Forests act as a natural barrier against soil erosion by stabilizing the soil with their root systems and preventing soil runoff.

Option 3: Building material - This is a direct use of forests because timber and other materials used for construction are obtained directly from the forest.

Option 4: Grazing - This is a direct use of forests because it involves allowing livestock to graze in forest areas for food.

In conclusion, option 2 (Checking soil erosion) is the correct answer as it represents the indirect use of forests.