Heard the term “well modified grain” today, what does that mean?
Practically, that a single infusion mash will suffice.
Yep. To expand a bit, modification indicates how easy it is to mash a grain. How much of the star h is solubulized. Great article here by John Palmer…http://www.homebrewersassociation.org/attachments/0000/1291/JFzym03-Extracting.pdf
They were modified in a well.
I just saw a presentation about malting by Deer Creek Malthouse. He described it as the amount of time the barley is allowed to germinate before being dried and kilned. The acrospires (ie- tiny baby plant) grows up the side of the barley grain, when it reaches the tip, the grain is fully modified. The grain is creating/or activating enzymes as it germinates (you know, like alpha amylase) so being fully modified means the entire grain has begun the germination process and is full of enzymes.
solubulized
Is that actually a word? Trying saying that after a couple.
It’s a test! 
A search result, modification has a graph about 2/3 the way down. the rootlets need to be about as long as the grain.
http://www.daltraining.eu/WebHelp/001_Raw_Materials/1.2.4._The_process.htm
The rootlet length and the acrospire length are indications of the level of modification but not the modification itself. Denny’s description is close. And Jimmy K touched on part of it as well.
The enzymes are protein molecules that are bound up with the starch in hard flinty bundles in the barley seed. Very stable and resistant to heat, drought, space travel, pretty much whatever.
When the germination proceeds the enzymes are liberated from the starch, in the process drilling holes in the structures and turning them mealy or friable.
If you take a sample of modern mega malt and chew is it’s pretty easy to crunch up entirely. That’s highly modified. If you try to do the same with a barley seed… well maybe make an appointment with the dentist first, just in case. It’s doable with strong teeth but not easy like with malt.
If you can find less well modified malt you will find some kernals are easy to crunch up on one end and still hard to chew at the other.
you can also take samples of the malt and drop them in a glass of water, a fully modified kernal will lie sideways on the surface of the water and float. An unmodified kernal will sink to the bottom and a partially modified kernal will float veritically with the unmodified end pointing down.
effectivly it means two things, As Brulospoher said, the proteins are broken down in a favorable way and completey so you don’t need to worry about a protein rest to prevent haze, and the starch is fully available so you don’t need to use a decoction to maximize extraction.