I didn't know until the current discussions that there was a technical name for this concept. But I sure know the phenomenon. Because most health insurance in this country comes through our employers, lots of people are trapped in jobs they would otherwise leave, if quitting wouldn't mean they would lose access to health care.
Individuals with what insurers consider disqualifying pre-existing conditions can only get insurance at all by sneaking in through some employer's group. Even if they get a new job with health coverage, coverage of their pre-existing condition may be excluded for some period. Pretty much, if you've ever been sick, you can only make sure you are covered by working for an employer with a group policy.
Some people might be able to get insurance in the individual market, if they leave their job. But it will be costly, probably cover little aside from catastrophic illness, and is precarious (it's easy for insurers to dump individuals who cost them money).
All of this means that a lot of people are stuck with their jobs in order to have insurance. This is called "job lock." For relatively healthy people, the permutations of job lock are probably the most offensive aspect of their experience with the current health insurance market.
Too many people are stuck in jobs they'd like to get out of in order to keep their insurance. They might want to do something else -- and it might benefit the economy if they could strike out as entrepreneurs -- but job lock means they won't.
Some employers may like the leverage that job lock gives them over employees who fear losing health insurance if they are laid off. But other, usually small, employers would be glad not to be in the difficult sideline business of having to make annual decisions about how to buy health care.
I think the President hits the wrong note by talking so much about how, under the proposed reform, if you now get health insurance through your employer, nothing is going to change. Many of us want things to change. If you want to mobilize us for reform, promise us we will be freed from job lock.