TSA pre-check is a pre-screening program that air travelers can use to bypass the normal security line.
It's not a huge time-saver (any more, used to be more) but gets you out of a few annoyances like removing shoes, unpacking toiletries, and a few other things. And the lines are usually a bit shorter and the ratio of regular traveller to new traveller is a bit better (less likely to get caught behind somebody who doesn't know the process and wastes everybody's time).
And they don't put you through the nudie scanner (or whatever it's called) which is important to some people. I have pre-check automatically through global entry (that basically lets you bypass immigration coming back into the US).
Just got ETA for the UK and you also have e-entry (which usually works) from the US and a number of other countries as well.
Usually true, though I don't believe that's guaranteed.
And all the lines at IAD have the new, high-powered luggage scanners (for carry-on, so still less powerful than the ones used for checked bags). So for those of that fly with film camera gear, we need a hand check with or without pre-check.
