If "Ignore" is checked, it will not affect the related product inventory.

for example:    
    Product A uses 1 x Material A
    Product B uses 2 x Material A

When the Ignore option of Material A is unchecked: Customers buy product A, the stock of material A will be reduced.

because the stock of material A reduced, the system will automatically calculate the inventory of Product B and modify the inventory of Product B.

When the Ignore option of Material A is checked: Customers buy Product A, the inventory of Material A will be reduced, but the inventory of Product B will not be changed