An input tax credit (ITC) is a credit a GST/HST registrant can claim for the GST/HST it pays on purchases and expenses used in its commercial activities. ITCs reduce the net tax the business remits: the registrant subtracts the HST it paid on inputs from the HST it collected on sales and remits only the difference.
For example, a business that collected $2,600 of HST on its sales and paid $1,040 of HST on its expenses claims the $1,040 as an ITC and remits the net $1,560. A registrant whose sales are zero-rated (such as an exporter) may pay HST on inputs without collecting any, producing a refund. Registering for GST/HST — even below the $30,000 threshold — is what unlocks the ability to claim ITCs.