I've heard stories of people who were employed by one or two employers over their lifetime. These stories are usually accompanied by terms like “job security”, which seem to imply that your ability to earn a living, and to reliably do so for many years into the future, is something that is given to you by your employer.
Oh what glorious times those must have been, because these days “job security” is all about your ability to replace your employer, not your employer's inability to replace you.
In the context of finance, liquid assets are those that can be quickly exchanged for cash with little to no loss in value. The same concept applies in the job market. Your liquid skills are those that will allow you to quickly get another job with the same pay and conditions. Your liquid skills are those that give you job security.
Before you can understand what liquid skills are, it pays to understand what the opposite is:
For example: you're a COBOL programmer, and the only person who knows how to keep the critical application running on an archaic IBM 360 mainframe at the heart of E Corp's accounting division. E Corp's CTO pays you megabucks to do it. Anywhere else, you will get paid exactly $0 for your mad COBOL skillz and your in-depth knowledge of E Corp's bespoke accounting application and big-endian mainframe architecture.
The longer you stay there, the more rare your skills become. The more valuable they are — at E Corp; the more worthless they become everywhere else.
Right up to the point where E Corp blows away the mainframe, replacing it with a Node.js-powered frontend to a BizTalk ESB - leaving you flipping burgers and talking about the good old days.
There are a few well known sources that track trending jobs skills:
TIOBE Index for popular languages.
ThoughtWorks Radar for technologies and platforms.
Gartner Trends for IT research and analysis.
Of course, to actually validate the liquidity of your skill set, you can't go past job sites as a way to gauge your current marketability:
Before you find yourself with a case of the Mumps that leaves you unemployable outside of your current position, it might be worth taking a few moments to list your liquid job skills and find out just how much modern job security you actually have.