It’s not like riding a bike: you can’t hop right back on and it doesn’t come back easily, even though you still might end up getting your groin impaled by the contraption you’re trying to control.
Skill rot is what happens when you don’t do something for a while. Just like muscle tissue, your skills slowly decline without regular use, no matter how good you were before. Skill rot sucks. And worse, it sets in fast. You might spend three to six months getting pretty good at something, whether its HTML programming or fine woodworking, and if you don’t do it for just a few months, poof, it’s pretty much gone. Even if you’ve spent years getting great at something, taking six months or a year off can have similarly devastating effects. The stuff you once knew how to do without even having to think now require a trip through google hell or over to the reference book.
So, why do we care? Well, other than maintaining your skills by regularly doing whatever it is you’ve spent time to get good at, you should think about what you want to devote your time to learning in the first place. Ya, learning some java so you could build that web app would be cool, but if you’re a mechanical engineer, and you’ve only got plans to build the one thing, it’s probably going to be a waste. Even if you get around to working on project number two, by the time you do, you’re almost sure to have forgotten everything you spent months learning to complete project number one. Better to hire somebody to do it for you and spend your time honing or acquiring skills that you’ll regularly use, and thus won’t soon lose.
The good news is, just like muscle tissue, the better you were at something, the more intimately you knew how things worked and why, the faster it comes back.
*Caveat: This thinking shouldn’t apply to anyone under 21, who, if they don’t yet have a passion, should try as many things as possible until they find that thing they can be passionate about.