Well, when folks use the term "IO'd out", I interpret that as meaning "I have maximized the performance of my build/powerset through set IOs and by slotting the build specifically to take advantage of them". Typically there are two main performance benefits for IO'd out characters. First is the aggregate enhancement value for Set IOs exceeds generic IOs which exceed SOs. Six-slotting a set like Crushing Impact gives the equivalent of having 9-10 SO's in the power. That's a substantial increase in effectiveness over per-issue 9 characters.
The second benefit is in the stacking Set IO bonuses. The variety here is staggering and allows a vast amount of customization for the same powersets. Typically these bonuses are designed to spackle over holes in powersets (like Psi Defense/Resistance for Invulnerability Tanks) or to enhance the main attributes of a powerset (i.e. extra damage and reduced recharge for Scrapper's attacks).
Take a look at Cap'n Canadian's guide to start with. PeterPeter's guide is also a good place to start.
Take a look in the guide in my signature. There are a lot of good links there for IOs and making influence to buy them.
Yup, what Garth said..
Also, you could probably get some good suggestions if you stated the build you wanted to IO 
perma jump is ---> /up 1
Ok, since i9, people have been going on about having their toons, "IO'd out." After viewing the builds people posted, it astounds me.
I'm way behind on all of this "proc'ing out your build" stuff, and I still use SO's compared to IO's. Anybody have any tips on how to "IO out" your build? Seems like you just pick random recipes and franken-slot them on stuff.
Mostly I would like to do this on my Mind/Emp troller, though he doesn't have the vast amounts of money some people seem to have.
NOTE I probably didn't use the "IO'd out" and "proc'ed out your build" lingo correct.