When creating a new tool, the harvest level is going to determine how hard blocks you are able to break using this tool. When creating a block, this value is going to determine which tools can break it/make it drop anything.
A block in Minecraft can have different harvest levels for different types of tools too.
Condition for the block to drop the items when broken is:
IF BLOCK HARVEST LEVEL <= TOOL HARVEST LEVEL
|Wood / Gold||Stone||Iron||Diamond|
|Harvest Level (Value)||0||1||2||3|