In addition to your monthly installment payments, the full outstanding amount is authorized (held) on your card to guarantee future payments. Authorizations are usually renewed every 17-21 days until your plan is paid off. Authorizations get smaller with each installment paid. The previous authorization is released once a new one is obtained. Authorizations show as 'pending' or 'reserved' on your account. If your plan is kept on track, these authorizations are not charged.
You can check your monthly payments as well as whether a re-authorization is due (when and for how much), in the Splitit shopper portal.
There are some variations to the above, depending on your retailer and the payment processor they use.
To see exactly how your plan works, log in to the shopper portal and under 'Plan Details' click on 'How Splitit Works?'. This will take you to a detailed explanation of your particular plan.