If you're suffering from high latency, low framerates, disconnections or similar issues, resetting your interface is worth trying before attempting more extreme measures. They're not talking about a simple /reload UI, which can be typed into the chat box, rather about renaming your Cache, Interface and WTF folders.
Why rename them? Don't you usually delete them? Well, you can certainly do that, but if the reset doesn't fix your issues, renaming them will mean that your settings can be restored to how they were before. As the video says, exit the game, and open the containing folder of your WoW installation. Rename the Cache, WTF, and Interface folders, to whatever you like. When you next start WoW, the folders will be recreated. If this fixes your problems, reinstall your addons one by one. You could, theoretically, grab installs and profiles from your old folders, but if something has become corrupted you risk replicating the previous issues.
A less extreme course of action than the one suggested in the video is to get an addon like Addon Control Panel that allows you to switch your addons on and off without logging out. Disable all your addons, then re-enable them one by one. You may find that one addon is the culprit! If this doesn't work, then your next step is the action recommended by Blizzard's team.
As always, keep your addons updated, either at their own sites or via Curse, WoWInterface or WoWAce. These "big three" addon sites are where most addon creators host their work, so other sites may not necessarily have the latest updates or correct versions.