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...)

The power armor radiation gauge simply gives you an idea of what radiation is present in the surrounding environment.

It's important to be aware that, while certain things like power armor and hazmat suits greatly reduce radiation damage, nothing will completely negate all rads. If your Geiger counter is clicking, you're probably taking some amount of radiation damage.

So, this provides you two things:

• General awareness of when radiation damage is being taken.
• An idea of where not to step out of your power armor. (Unless you're wearing a hazmat suit underneath.)

Another thing to be aware of is that damage and max HP reduction from rads is not based on static values in the same way most other damage is. Each rad absorbed takes out 0.1% of your maximum health, regardless of how high or low your natural maximum HP value is.

Rad absorption that's displayed in a numeric form is generally in terms of "rads per second". The greatest available radiation resistance (such as what you can get with power armor) reduces most sources of radiation to "<1" rads per second.

This means you'll usually have somewhere north of 16 minutes and 40 seconds worth of survival time, before you'll have to use Aids or die, while the Geiger counter inside your power armor is clicking away. Certainly, this should be more than enough for most scenarios. But it's still important to be aware that it is a finite period.

One particularly annoying bit about the power suit's radiation gauge however is that it doesn't actually show you how much damage you're taking. Usually, it's extremely low ("<1 rads") but it would be nice to actually see that value so you can be certain.

The suit's HUD also doesn't give you a clear indicator of how much of your maximum health is currently unavailable due to radiation damage already taken. (Well, apparently there is an indicator but it's really easy to miss.) This can be particularly annoying when you are trying to judge how to effectively use your Aids to restore available HP or remove absorbed radiation.

To work around these limitations, you can go into VATS at any time by holding the normal VATS button for a half-second or so. (This works in and out of combat, regardless of whether there's anything around that can be targeted.) There, you'll see the normal health bar and radiation indicators as appropriate.