Ease is the space between your body and the garment you are wearing. It determines if you can move comfortably and how your garment looks to others. Is it tight or over sized, does it flatter you or do you look lost inside your clothing.
I have read everything I can find on ease.I've compared all of the various charts. I've read what both knitters and sewers have written in an attempt to find a clear and simple way to teach this concept to my students. I've taken pattern drafting classes, courses from other knitters and I have a long history of both sewing and knitting.
Most of the charts look like the one I've copied in below from http://www.yarnstandards.com/sizing.html
Actual chest/bust measurement or less
6"/15cm or more
The first thing you will notice is that this information is pretty vague. Also notice that there is no numerical measurement of negative ease. The charts don't give any information about the hand of the fabric either. Hand refers to the feel of the fabric in terms of stiffness or drape. Think about the difference between a light as air silk mohair, a heavy silk ribbon yarn that drapes and 100% wool that has been felted. As you can imagine each of these has very different ease requirements. The general rule is that the stiffer the fabric the more ease that is required. But how much exactly?
When sewers buy a pattern meant to be used for knit fabrics the back of the envelope includes a stretch scale. The scale is there so that you can compare the fabric you plan to use against what the designer intended. It determines that you will have the correct amount of ease. Even here it is assumed you picked your right size by actual measurements not by the size number of ready to wear clothing or the size you hope to be after your next diet.
Unfortunately there is no comparable scale for hand knitters. Often we don't even have access to the original yarn choice of the designer. Yarn substitutions are common and your LYS will do the best they can to advise you. Your swatch is where the most information comes from but you don't have that until after you buy your yarn and if you are a novice knitter you don't yet have the experience to get all the information you need from your swatch.
When you are buying a pattern the designer has tried to work all of these factors out in advance for you. Unfortunately even our best attempts often fail our audience. The individual knitter may have difficulty getting the recommended yarn, getting gauge or they may choose to knit the wrong size if the pattern is not interpreted correctly.
One of the reasons for this has to do with individual preference. Some of us like close fitting garments and some of us don't. As a knitter you need to figure out what you personally like and the best way to do that is to look in your own closet. Measure yourself, measure a few of your favourite garments and compare to the chart. It will give you a starting point when choosing which size of the pattern you are going to knit and what adjustments you need to make to that pattern.
It also helps to recognize that you will need to test out ease on a number of projects to develop a strong sense of what is correct for you and the intended wearer of any garments you knit.