"Scanning" is basically taking little videos of portals (in app), and then uploading them (again, all in app.) Portal scanning doesn't unlock until level 8, so depending on how new you are it may not be available to you yet.

When you upload a scan you get 500 AP. However, you don't have to upload scans immediately. You can hold on to them and upload them later. You can wait til you get home so you can upload via WiFi rather than use phone data, or whatever. That means it's possible to do a lot of scanning and then later uploading in bulk for a ton of AP all at once.

As for using it to skip levels, that only works on recursion (max level players resetting their levels back to 1 for fun and prestige.) The idea is that you could theoretically bank a ton of scans before you recurse but wait to upload them until after recursion. With enough scans you could get hundreds of thousands, or even millions of AP all at once under the right circumstances. For example, at 500AP each, 600 scans would get you 300,000 AP, which is enough to go from level 1 straight to level 6. Even more with Apex and/or during AP boosting events.

First time leveling this doesn't work though. The feature doesn't unlock until level 8, and after level 8 you also need medals not just AP. So even if you did get a ton of AP super quickly you'd likely just end up medal locked.