It means that the item is "better"*

So I killed a bunch of raiders and they dropped me some loot:

But before picking it up and after taking the screenshot, I took off all of my armor and stood naked in the street to do a comparison:

As you can see, before when I was fully geared up, the items weren't upgrades. However, after I had removed my gear, the items were upgrades.

Now, from my testing it doesn't seem as clear as this and you can find instances where this will tell you an item is better when in fact it isn't but this symbol is intended to indicate that the item you're looking at is better than the one you already have equipped. Here is an example:

That gun I'm holding in the background is a Big Boy. It does an unreasonable 486 damage in comparison to the Combat Rifle (which the game tells me is better) which does 34, however the Combat Rifle is better in every other statistic (except Value) and thus with a much higher rate of fire it does more damage per second on a single target, not taking into consideration the massive area of effect the Big Boy causes - the game believes it to be a "better" weapon.

Such examples are mainly context based or class based, for example comparing a pistol to a sniper rifle or comparing power armor to regular armor or looking at armor that provides a damage resistance bonus over your current gear when you're wearing a radiation suit and chilling out with the Children of Atom.

Personally I'd take the plus as indication the item is worth a look at in comparison to what you currently have but that in some instances you'll find that your current item is better for how you're playing (whether that's an item specific perk or comparing different weapons classes or whatever...)

No, the T-45 upgrades are not transferable to the T-60.

I have both, and have tried this. Additionally, if you remove the mods from the T-45 and look at them in your inventory, you'll see that they are labeled T-45.