It's a "Top Secret Clearance," a certificate saying that you are trustworthy to perform top secret government work. Based on what I've heard from others, getting one includes taking a lie detector test, having family members, neighbors, and colleagues interviewed, and hopefully not too many other probing things. My impression is that any warm-bodied US citizen who doesn't have much of a record, and who knows the right things to say, can get a TSC unless their nationality gets in the way.

You get a clearance by having your employer or client sponsor you. It inactivates when you switch jobs, but can be reactivated fairly easily within 24 months. Beyond that, you need to be re-investigated. Standard Form 86 [1] lays out the information you need to provide.

[1] https://www.opm.gov/forms/pdf_fill/sf86.pdf