That's what I used to do for a living, for more than two decades.
SIGINT is the intercept and analysis of signals, usually electronic but not always. Most of the time those signals are some form of communication, between humans, between machines, between instruments, and any of myriad variations in between. But SIGINT may also be derived from non-communications signals such as radar or other sensors systems and perhaps other things I'm not going to talk about. It is a vast, vast field of intelligence, broken up into many, many sub-specialties, and it changes constantly following advances in the state of the art. It can involve the intercept of rudimentary decades old forms of communication such as morse code sent via simple on and off carrier wave, to communications so sophisticated that you could barely even comprehend the technology without enormous processing power to assist you. The modern US signals intelligence system is the end result of literally centuries of effort, sacrifice, and expense -- most of which you've never heard of and will likely never know. This source of intelligence, like other intelligence, can give our nation and our allies advantage not just in war, but also in diplomacy, in economics, in just about everything. And yes, it can most certainly be abused (and has been, Watergate being an infamous example and one of the few we can sort of talk about) which is why it is so very, very important to have systems in place to ensure control, protection, and release of that information.
I'm going to focus primarily on SIGINT for the rest of this essay, because that's my area of experience and expertise.
I'll leave the nuclear stuff to those who are experts in that area.
Now, while it's true that a president can declassify most anything SIGINT related, (it's my understanding he cannot declassify information regarding nuclear programs) it's extremely problematic for any president to do so in any sort of impulsive or cavalier fashion.
The fallout (yep, I did that on purpose) could be devastating to national security.
