A little vague on the descriptions aren't they?
The adapter is just a male BNC on one end and a female BNC on the other. The '50 ohm' refers to the impedance. If you measured the resistance between the center and outer conductor, you'd see an open circuit. What's the point? It's a bit unusual to see such an adapter in the BNC family, but it's quite common on higher frequency devices where it's known as a 'connector saver'. You put one on a device that uses a small, somewhat delicate SMA connector (or a higher frequency version like 3.5 mm, 2.92 mm, etc.) so that if it becomes damaged you just change the connector saver rather than have to change the connector on your device. That could cost thousands to have the device repaired and recalibrated.
The terminator is also known as a feed-thru or feed-through terminator. As shown on the datasheet, there is a 50 ohm resistor between the center and outer conductors. As Dave discussed in the video, it's used to terminate the cable when connecting to a device with a high input impedance. It's most often used with a scope because most other devices already have 50 ohm inputs. Some scopes have the option built in.
The cost of these terminators depends on the performance and the brand. If you search the forum you'll find a couple of threads that discuss some cheaper versions that are available online. Whether the cheaper versions are acceptable will depend on what frequencies you work with. Below 100 MHz, the cheaper versions should be fine for all but the highest precision work.
Ed