This new feature is located under Campaigns > Edit This Campaign > Order Settings > Cart Settings.
Cart Settings is where you can activate or deactivate the Cart feature in your campaign. Activating the Cart in a campaign will allow your supporters to complete multiple activities with a single transaction. For example, a supporter can register for an event, make a donation, and purchase a tee shirt in a single transaction. This means they will enter their personal details and payment information once, instead of 3 times (separately for every action). This also means they will receive a single emailed receipt/thank you/confirmation containing all of the details pertaining to those activities.
Activating the Cart feature also gives you the option to set a maximum shipping cost per order, set a waiver for the entire order, set discount codes/coupon codes either per item or for the entire order, as well as apply custom fields to the entire order.
By default, the Cart feature is disabled for all new campaigns. Leaving the Cart disabled will require your supporter to complete a separate transaction for every tool/engagement they want to take on this particular campaign. Since most campaigns contain only one tool/engagement, this is usually the best setting. If you do have a multi-tool campaign in which you'd like to keep the Cart disabled, you can still set up Chain Actions for each Tool to urge a supporter to complete a subsequent action.