What does the word switching mean?

It has nothing to do with turning the radio on and off. It describes the electronic circuit in the power supply that converts the line voltage alternating current to the low voltage direct current that the radio needs. It does this conversion by rapidly turning on and off at the appropriate times within the AC supply sine wave to grab small packets of power to assemble the DC output. This is used instead of a transformer and rectifier because it uses less power when the radio is off. This type of power supply is mandated by Federal energy standards.

What is the difference between this adapter and the sangean adp-prd18 switching power ac adapter for models pr-d18, pr-d4w and cl-100?

The difference is the amount of current that it can provide. The adapter for PR-D19/PR-D14 is rated at 1 amp whereas the PR-D18/PR-D4W/CL-100 is rated at .4 Amp. You need to use the power adapter that is rated for the device you will use it with. That rating should be printed on the back of the device.

Can i just use a usb to a barrel connector cable for about four dollars instead of a $22 adapter?

Do whatever you want! If you use usb/barrel adaptor, does your phone charger provide enough amps to power whatever you’re running? I needed a dedicated adapter... it was also about $9 when I bought it, not $20!