InSitu Sales pulls barcode data from Quickbooks. You can also manually add barcode data on the web app by going to Products/Items, clicking on the item, and entering the barcode number.
Barcode scanners typically have their own custom configuration for syncing with mobile devices. If you have one that operates on bluetooth, you can enable bluetooth via the Settings on your phone, and then add the barcode scanner device via the list of local devices.
When viewing an item list on the mobile app, scanning will act as a filter, displaying the corresponding item.
Here is an example for iOS users with the CT-20 Barcode scanner: