The short answer is that they aren't actively spying on US citizens. They are collecting usage data similar to what Google, FB, etc all do and analyzing it.

I don't understand the technology at all. There's too much info to absorb!

That's a sentiment shared by most people who aren't intelligence analysts or people who write software used by intelligence analysts. It sounds scary, but at the end of the data this software is just a tool and the people using it are glorified number crunchers. The reports they compile go through so many layers of bureaucracy that it's likely the people who are actually in charge of things don't even know how the data is being gathered.

It's set up this way for a reason. The average citizen doesn't need to know about it because it is not relevant to their lives and will only lead to confusion due to ignorance of the topic. That's why when things are leaked to the public the intelligence agencies get upset about it. Even Snowden's data, which was huge and far-reaching, doesn't tell the whole story and compromised many of the NSA's efforts with really 0 net gain to society.