Item Power is essentially the strength of your gear. The more IP the more damage/healing/shielding/crowd control will your weapon do. Head/Shoe slots will increase your hitpoints while chestpieces increase health and resistances. 100 Item Power equals to about a 9% increase in damage and defensive stats so having more increases your chances in a lot of fights.
Whether you want to have 50,100 or even more spec in an item really depends on your goals. There's no clear definition of "low or high ip" but depending on the content a lot of players will say that anything below 1200 IP may be considered low. PvP/PvE groups in the blackzone usually run around 1300IP+ but it really depends there's also some that have a lot of fun running 4.1.
Your spec can have a big impact on your item power. If you get for example 100 spec in the whole tree (often called 700/700 or 800/800 you can get 300 IP and more added to that specific equipment which equals around 3 tiers of gear so so...
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