Some jobs in engineering, technology, government, or other classified work require a security clearance. There is a procedure to go through to get it. Typically an employer will pay for it. It can cost about $5000 for a "secret" clearance and $10, 000 for "top secret". It will involve filling out a great deal of paperwork and having one or more Department of Defense briefings about regulations such as procedures to follow when dealing on the job with persons of foreign background, etc.
There are multiple levels depending on the amount of information that you will have access to. All it really means is that you are safe to see information that could be considered sensitive, at the least.
In general, they are not easy to get. Even a relatively low level security clearance can take 6 months to a year to get, and even longer. If you have contacts with foreign nationals, or dated (or married) someone from another country, it will take even longer. Last I heard from someone who was in the process of getting a clearance was that about 50% of applications for even low level clearance are rejected.
I heard they check your credit history.
Getting a security clearance could take a LONG time. Depends on the level of clearance needed. For SECRET, about 6 months. TOP SECRET and higher can take 18 months or more.
If you have non-US citizen family members, extensive foreign travel or residence, it can delay the process further as they try to run investigations overseas.
If none of this applies, you've never been in trouble with the law, no drugs, etc., the process could move more quickly.