tire size

That will fit just fine. The 110 is just a wider tire. The first number refers to overall tread width the second I believe is sidewall height and the third is your rim size. So as long as you got a tire for a 19" rim, which it looks like you did, you are all set. You will just have a wider tire on there now.

100 is the section width of the tire in millimeters.
90 is the aspect ratio, which is 90% of the section width in this case.

So the tire is 100 mm wide, with sidewalls that are 0.90*100 tall = 90 mm tall

The 19 is the rim size but you already had that figured.

While the tire will fit (provided no interference with the swingarm), it is designed for a wider rim than is on your bike, so the profile of the tire will be distorted slightly. This may reduce traction somewhat, and the center knobs will wear a little faster. That tire will also be slightly heavier, and load the engine a bit more. It won't hurt the bike at all, but the proper size rim would bb better to run it on, or the correct size tire for the rim.

To clarify TimberPig's post, the section width is only accurate if the tire is mounted on a rim that has the tire manufacturers recommended width. The actual width of the tire mounted on a 1.85 inch wide rim will NOT be the same as the width of the same tire mounted on a 2.15 inch rim.
The aspect ratio is the height of the sidewall, written as a percentage of the section width.
The extra width may also allow the tire to roll sideways a little more than you are used to, this will probably feel like the rear end is wallowing around the turns.
Sorry JST, but the first number has nothihng to do with tread width, but it is OK to run the tire, as long as it doesn't rub.

Clarify for me what section width is. I have always been under the impression that the first number refers to the width of the tire so from one side of the tread pattern to the other or basically from bead to bead. However, I am apparently wrong in my exact understanding of it otherwise you wouldnt have pointed it out.

Thanks Timperpig I appreciate it. Now I know exactly how it is measured but atleast I wasnt way off with my understanding. I have been going off of the thought that it was the tire width, which it essentially is, just not measured at the tread or bead. The way FruDaddy made it sound it was like I was way off in lala land. I am sure that Troyboy appreciates you clarifying the specifics of each number though so now he is more knowledgeable about it. I should have gone into more detail explaining it.

Troyboy I am currently running a 110 on a 1.85 rim on my KDX and it works fine and clears the swing arm without a problem. Granted your bike is different, but the size difference between the widths of the two tires is not substantial enough to cause a fit problem as far as clearance goes or mounting. The points Timperpig has brought up as far as wear and weight are valid. But you arent going to notice a power loss on the seat by running this tire.

Didn't mean to make it sound like you were so far off, just wanted to make sure that nobody was going to break out a ruler and say that there is plenty of room. Timber's original definition was correct, I was trying to add clarity not confusion. In the auto industry, you will often find differences in tread width of an inch, with the tires being the same measured size.
Also, I have seen a 10mm size increase cause a rub on a street bike, probably won't be an issue on the KX125 though.