Shopify POS is Shopify's point of sale: the app your staff use at the counter, connected to the same products, stock and customers as your online store.
Sell a dress in the shop and the website's stock drops at the same moment, and the other way round. That only works when products, locations and stock are set up properly: most "inventory not syncing" problems come from duplicate products, a missing location, or stock counted against the wrong one.
It runs on a phone or tablet, or on Shopify's own POS Terminal. Cards are taken with Tap to Pay or a compatible card reader, and Shopify also offers a POS Hub, a barcode scanner and a tablet stand for busier counters.