Minecraft wiki entry for Redstone_Dust, associated text: from an adjacent power component or a strongly-powered block
from the output of a redstone repeater or redstone comparator
from adjacent redstone wire. The powering dust can be a level higher or lower, but with restrictions:
Redstone dust can be powered by redstone dust that is one level lower, or on an opaque block one level higher. A transparent block cannot‌[Java Edition only] pass power downward.
The block "between" the two dust blocks must be air or transparent. A solid block there "cuts" the connection between the higher and lower dust.