Input Tax Credit (ITC) is the mechanism that prevents tax-on-tax under GST. A business pays GST on its purchases (input tax) and collects GST on its sales (output tax); it pays the government only the difference, claiming credit for the input tax.
ITC is conditional: the supplier must have actually reported the invoice, the goods or services must be received, and the credit must not be blocked or restricted. Reconciling purchases against the auto-drafted GSTR-2B is how businesses protect their eligible ITC.