It is easy for us to understand that getting into the top three positions for a 45% keyword is going to be more difficult than getting there for a 28% keyword. So, at first glance this is really easy to comprehend.
That is the basic value in the keyword difficulty tool.
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To understand this tool fully, you need to build a few websites that each have some content out on the web. Then you will see a piece of your content ranking for a keyword and know what it took to produce that. Now you have "experience" that you can use as a gauge. Now you have a number and it will be like holding a rock in your hand, hefting it a bit and having some idea of APPROXIMATELY what it weighs.
However, keep in mind that this scale is from 1% to 100% and it is probably not linear. A rock that gets a keyword difficulty of 58% probably weighs about ten times as much as one with a 48%.
The scale is logarithmic (or something close to that - but definitely not linear).
Now.. you asked about easy vs hard and how long it takes to rank. That depends upon the amount of resources that you have to put into the battle. If I have a powerful site, I can toss up a page and it might immediately be competitive and on the first page of the SERPs for a 45% keyword. EASY.
But you put the same content on your site, which is new and has not developed a strong track record, and it might not be visible in the first five pages of the SERPs. HARD.
The difference is that I have the resource of a strong website behind me and that makes the KW easy for me. You don't have the luxury so the same KW is hard for you.
Other differences in resources can be budget for site promotion, relationships with other webmasters, and an enthusiastic tribe of website visitors who will tweet... "Chapman nails it again! Woohoo!" That can be like throwing gasoline on a fire.
Another difference is the quality of the content. My strong site might get that first page of the SERPs for a 45% keyword with junk content. But you write something really impressive that attracts links, likes, tweets, and attention. Although you rank initially on page five of the SERPs. People who see your content share and link to it at a much faster rate than the junk article on my site... so over time your rankings will climb. My page will eventually be defeated as superior content is published on other sites.
The bottom line... SEO is a "battle of resources". Knowledge, talent, wit, verve and dedication to quality are amazing resources in the long term. But the resources of a powerful site, a huge budget, established relationships and an enthusiastic tribe give Goliath a huge and immediate advantage - even if undeserved.
So, easy vs hard is relative... but the number scale ranks keywords in their approximate order of "resources required" - which we could substitute for the word "difficulty".