Point of sale marketing refers to the promotional activities carried out by brands at the point of sale - a typical example is near the cash register/ checkout queue. This concept capitalizes on the impulse purchase of certain products observed across a majority of shoppers. Generally it involves attractive displays to grab attention of the captive crowd in billing lines.
It means point of sale, or point of service. It’s a bit of a misnomer, because that’s only part of what it covers.
Tills are computers. Imagine buying yourself a USB cash drawer. Now your computer (with the right software on) is a POS.
The POS in your local shop is much the same thing, with a few more enhancements - barcode scanner (essentially just a specialised keyboard), barcode database, several tills linked, an automated ordering system, an analytics package, and maybe some self service checkouts.